Surface features for imaging directional backlights

ABSTRACT

An imaging directional backlight apparatus includes a waveguide and light source array for providing large area directed illumination from localized light sources. The waveguide may include a stepped structure in which steps may include extraction features optically hidden to guided light, propagating in a forward direction. Returning light propagating in a backward direction may be refracted, diffracted, or reflected by the features to provide discrete illumination beams exiting from the top surface of the waveguide. Viewing windows are formed through imaging individual light sources and defines the relative positions of system elements and ray paths. Alignment of the waveguide to mechanical and optical components may be provided by surface relief features of the waveguide arranged in regions adjacent the input surface and intermediate the light emitting regions of the light sources. Efficient, uniform operation may be achieved with low cross talk for application to autostereoscopic and privacy modes of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Appl. No.62/255,270 entitled “Wide angle imaging directional backlights” filedNov. 13, 2015 (Attorney Ref. No. 390000), which is herein incorporatedby reference in its entirety.

TECHNICAL FIELD

This disclosure generally relates to illumination of light modulationdevices, and more specifically relates to light guides for providinglarge area illumination from localized light sources for use in 2D, 3D,and/or autostereoscopic display devices.

BACKGROUND

Spatially multiplexed autostereoscopic displays typically align aparallax component such as a lenticular screen or parallax barrier withan array of images arranged as at least first and second sets of pixelson a spatial light modulator, for example an LCD. The parallax componentdirects light from each of the sets of pixels into different respectivedirections to provide first and second viewing windows in front of thedisplay. An observer with an eye placed in the first viewing window cansee a first image with light from the first set of pixels; and with aneye placed in the second viewing window can see a second image, withlight from the second set of pixels.

Such displays have reduced spatial resolution compared to the nativeresolution of the spatial light modulator and further, the structure ofthe viewing windows is determined by the pixel aperture shape andparallax component imaging function. Gaps between the pixels, forexample for electrodes, typically produce non-uniform viewing windows.Undesirably such displays exhibit image flicker as an observer moveslaterally with respect to the display and so limit the viewing freedomof the display. Such flicker can be reduced by defocusing the opticalelements; however such defocusing results in increased levels of imagecross talk and increases visual strain for an observer. Such flicker canbe reduced by adjusting the shape of the pixel aperture, however suchchanges can reduce display brightness and can comprise addressingelectronics in the spatial light modulator.

BRIEF SUMMARY

According to the present disclosure, a directional illuminationapparatus may include an imaging directional backlight for directinglight, an illuminator array for providing light to the imagingdirectional backlight. The imaging directional backlight may include awaveguide for guiding light. The waveguide may include a first lightguiding surface and a second light guiding surface, opposite the firstlight guiding surface.

Display backlights in general employ waveguides and edge emittingsources. Certain imaging directional backlights have the additionalcapability of directing the illumination through a display panel intoviewing windows. An imaging system may be formed between multiplesources and the respective window images. One example of an imagingdirectional backlight is an optical valve that may employ a foldedoptical system and hence may also be an example of a folded imagingdirectional backlight. Light may propagate substantially without loss inone direction through the optical valve while counter-propagating lightmay be extracted by reflection off tilted facets as described in U.S.Patent Publ. No. 2012/0127573, which is herein incorporated by referencein its entirety.

Directional backlights provide illumination through a waveguide withdirections within the waveguide imaged to viewing windows. Diverginglight from light sources at the input end and propagating within thewaveguide is provided with reduced divergence, and typically collimated,by a curved reflecting mirror at a reflecting end of the waveguide andis imaged towards a viewing window by means of curved light extractionfeatures or a lens such as a Fresnel lens. For the on-axis viewingwindow, the collimated light is substantially parallel to the edges of arectangular shaped waveguide and so light is output across the entirearea of the waveguide towards the viewing window. For off-axispositions, the direction of the collimated light is not parallel to theedges of a rectangular waveguide but is inclined at a non-zero angle.Thus a non-illuminated (or void) outer portion (that may be triangularin shape) is formed between one edge of the collimated beam and therespective edge of the waveguide. No light is directed to the respectiveviewing window from within the outer portion and the display will appeardark in this region. It may be desirable to reduce the appearance of thedark outer portions for off-axis viewing positions so that more of thearea of the waveguide can be used to illuminate a spatial lightmodulator, advantageously reducing system size and cost.

In general with this and related imaging directional backlight systems,not all the backlight area may be usable due to vignetting at highangles. Modification of the system may overcome this limitation byintroducing light into regions that are void. Such modified illuminationapparatus embodiments may lead to increased brightness, localindependent illumination and directional capabilities.

According to a first aspect of the present disclosure there is provideda directional backlight comprising: a waveguide comprising first andsecond, opposed guide surfaces for guiding light along the waveguide andan input surface extending between the first and second guide surfaces;and an array of light sources arranged at different input positionsalong the input surface of the waveguide and arranged to input inputlight into the waveguide, the light sources having light emittingregions that are spaced apart, the waveguide further comprising areflective end for reflecting input light from the light sources backalong the waveguide, the second guide surface being arranged to deflectthe reflected input light through the first guide surface as outputlight, and the directional backlight being arranged to direct the outputlight into optical windows in output directions that are distributedlaterally in dependence on the input positions along the input surfaceof the light sources that inputted the input light, wherein thewaveguide further comprises at least one surface relief feature formedeither on at least one of the first and second guide surfaces in alocation adjacent the input surface and intermediate the light emittingregions of the light sources, or on the input surface intermediate thelight emitting regions of the light sources.

Said location of the surface relief feature may be within a regionbounded by: a portion of the input surface intermediate the lightemitting regions of a pair of adjacent light sources, and a pair ofintersecting notional lines that extend from the respective edges of thelight emitting regions of the pair of light sources that are adjacentthe portion of the input surface, to the respective sides of thereflective end that extend between the first and second guide surfaces.The surface relief feature may be a mechanical fixing feature. Themechanical fixing feature may be fixed to a further component of thedirectional backlight.

Advantageously, mechanical registration of the waveguide to otheroptical and mechanical components of the system may be provided.Mechanical registration may be achieved conveniently in the region ofincreased thermal expansion.

A directional backlight may further comprise a rear reflector comprisinga linear array of reflective facets arranged to reflect light from thelight sources, that is transmitted through the plurality of facetssecond guide surface of the waveguide, back through the waveguide toexit through the first guide surface into said optical windows, the rearreflector being said further component to which the mechanical fixingfeature is fixed.

Advantageously optical artifacts arising from movement of waveguide tothe rear reflector may be reduced. Further, the rear reflector may beconveniently aligned with other components of the mechanical and opticalsystem.

The surface relief feature may be a protrusion. Advantageously lowvisibility of input light coupling region and reduced cross talk forautostereoscopic and privacy modes of operation may be achieved.

The surface relief feature may be a recess. Advantageously increasedmechanical strength of the mechanical alignment may be achieved.

The surface relief feature may be arranged to remove from the waveguideat least some of the reflected light that is incident thereon afterreflection by the reflective end. Advantageously cross talk due to backreflections from the input surface may be further reduced.

The surface relief feature may be an identification mark. Advantageouslytraceability of components may be achieved without degradation to theoptical path.

The input surface may be an end of the waveguide that is opposite to thereflective end. The input surface may be a side surface of the waveguideextending away from the reflective end. The first guide surface may bearranged to guide light by total internal reflection and the secondguide surface may comprise a plurality of light extraction featuresoriented to direct light guided along the waveguide in directionsallowing exit through the first guide surface as the output light andintermediate regions between the light extraction features that arearranged to guide light along the waveguide. The second guide surfacemay have a stepped shape in which said light extraction features arefacets between the intermediate regions. The light extraction featuresmay have positive optical power in a direction between the side surfacesof the waveguide that extend between the first and second guidesurfaces. The reflective end may have positive optical power in adirection extending between the sides of the reflective end that extendbetween the first and second guide surfaces.

According to a second aspect of the present disclosure, a directionaldisplay device may comprise: a directional backlight according to thefirst aspect; and a transmissive spatial light modulator arranged toreceive the output light from the waveguide and to modulate it todisplay an image.

According to a third aspect of the present disclosure, a directionaldisplay apparatus may comprise: a directional display device accordingto the second aspect; and a control system arranged to control the lightsources.

Advantageously a directional display may be provided to achieveswitchable directional operation including autostereoscopic, privacy,wide angle, high luminance, night-time and power savings functions.

According to a fourth aspect of the present disclosure a directionalbacklight may comprise: a waveguide comprising first and second, opposedguide surfaces for guiding light along the waveguide and an inputsurface extending between the first and second guide surfaces; an arrayof light sources arranged at different input positions along the inputsurface of the waveguide and arranged to input input light into thewaveguide, the light sources having light emitting regions that arespaced apart, the waveguide further comprising a reflective end forreflecting input light from the light sources back along the waveguide,the second guide surface being arranged to deflect the reflected inputlight through the first guide surface as output light, and the waveguidebeing arranged to image the light sources so that the output light fromthe light sources is directed into respective optical windows in outputdirections that are distributed laterally in dependence on the inputpositions of the light sources; and at least one strip adhered to atleast one of the first guide surface and the second guide surface of thewaveguide and extending therealong adjacent to the input surface, thestrip being arranged to reduce reflection of light incident thereon frominside the waveguide.

The light sources may have light emitting regions that are spaced apart,and the strip may extend along at least one of the first guide surfaceand the second guide surface across both locations adjacent to the lightemitting regions of the light sources and locations intermediate thelight emitting regions of the light sources. The strip may extend alongonly a part of at least one of the first guide surface and the secondguide surface. Said part of at least one of the first guide surface andthe second guide surface along which the strip extends may be offsetfrom the center of the input surface. The strip may be an adhesive tape.The strip may be an adhesive material. The strip may have a refractiveindex that differs from the refractive index of the waveguide by no morethan 0.02.

Advantageously in a privacy mode of operation such a display can providereduced luminance for off axis viewing positions. Further the degree ofluminance reduction can be controlled by means of control of width ofthe strip. Further light loss for input light can be reduced by controlof size and location of the at least one strip.

The strip may have a refractive index that differs from the refractiveindex of the waveguide by no less than 0.08. Advantageously input lightstreaks may be reduced in intensity for small loss of head on luminance.Said part of at least one of the first guide surface and the secondguide surface along which the strip extends may be across the center ofthe input surface. Advantageously the light streak visibility for offaxis viewing positions in Privacy mode of operation may be reduced.

A directional backlight may further comprise a support which supportsthe array of light sources and may have a portion extending past theinput surface of the waveguide across the first guide surface or thesecond guide surface of the waveguide, and wherein the at least onestrip may comprise at least one strip adhered to the support and to oneof the first guide surface and the second guide surface of the waveguidefor holding the waveguide in position relative to the light sourcessupported on the support. The strip may be adhered to the support and toone of the first guide surface and the second guide surface of thewaveguide. The strip may be adhered to the support and to the first orsecond guide surface of the waveguide.

Advantageously the waveguide may be provided in a substantially fixedalignment with an array of light sources. Further the alignment meansmay have further function of reducing light for privacy viewingpositions and/or light streaks.

A directional backlight may further comprise at least one further stripprovided on the other of the first guide surface and the second guidesurface of the waveguide and extending therealong adjacent the inputsurface, the further strip may also be arranged to absorb light incidentthereon from inside the waveguide. Advantageously the level of privacyand streak luminance may be further increased for waveguide regionsoutside of the active area, achieving reduced bezel width.

The at least one strip may comprise at least one strip adhered to thefirst guide surface of the waveguide and at least one strip adhered tothe second guide surface of the waveguide. The strip may be absorptiveof light, whereby the strip reduces reflection of light incident thereonfrom inside the waveguide by absorbing that light. The strip may beabsorptive of light throughout the wavelength range of the light fromthe array of light sources. Advantageously scatter within the strip maybe reduced.

The strip may be transmissive of light, whereby the strip reducesreflection of light incident thereon from inside the waveguide bycoupling that light out of the waveguide. The support may be a flexibleprinted circuit. Advantageously conventional adhesive tape materials maybe used, reducing cost and complexity.

A directional backlight may further comprise a rigid holder portion towhich the support may be attached. The support may be a rigid holderportion. A directional backlight may further comprise a resilient memberprovided behind the light sources and resiliently biasing the lightsources towards the waveguide.

Advantageously longitudinal alignment of the array of light sources andwaveguide may be achieved in cooperation with improvement of privacy andlight streak luminance.

According to a fifth aspect of the present disclosure a directionalbacklight may comprise: a waveguide comprising first and second, opposedguide surfaces for guiding light along the waveguide and an input endcomprising an input surface extending between the first and second guidesurfaces; an array of light sources arranged at different inputpositions along the input end of the waveguide and arranged to inputinput light into the waveguide, the light sources having light emittingregions that are spaced apart, the waveguide further comprising areflective end for reflecting input light from the light sources backalong the waveguide, the second guide surface being arranged to deflectthe reflected input light through the first guide surface as outputlight, and the waveguide being arranged to image the light sources sothat the output light from the light sources is directed into respectiveoptical windows in output directions that are distributed laterally independence on the input positions of the light sources; a holder portionextending across the light sources and the waveguide, the holder portionholding the light sources and the waveguide in position relative to eachother; and a resilient member provided behind the light sources andresiliently biasing the light sources towards the input end of thewaveguide.

Advantageously longitudinal alignment of the array of light sources andwaveguide may be achieved in arrangements where it may be undesirable toprovide an adhesive strip between the waveguide and a support.

The directional backlight may further comprise a stop extending from theholder portion behind the resilient member, the resilient memberengaging the stop. The stop may be an integral part of the holderportion. The directional backlight may further comprise a support whichsupports the array of light sources, the support being attached to theholder portion. The support may be a printed circuit. The printedcircuit may be a flexible printed circuit.

Advantageously, the alignment and force on the light sources may beprovided by cooperation of the stop and the resilient member, to reducedamage during dropping of the directional display and other high impactevents.

The support may have a portion extending past the input end of thewaveguide across the second guide surface of the waveguide, and thedirectional backlight may further comprise a light-absorptive adhesivestrip adhered to the support and to the second guide surface of thewaveguide for holding the waveguide in position relative to the lightsources supported on the support, the light-absorptive adhesive stripextending along the second guide surface waveguide adjacent to the inputend. Advantageously longitudinal alignment of the array of light sourcesand waveguide may be achieved in cooperation with improvement of privacyand light streak luminance.

According to a sixth aspect of the present disclosure a directionalbacklight may comprise a waveguide comprising first and second, opposedguide surfaces for guiding light along the waveguide, an input surfaceextending between the first and second guide surfaces for receivinginput light, and a reflective end for reflecting input light from thelight sources back along the waveguide; an array of light sourcesarranged at different input positions along the input surface of thewaveguide and arranged to input the input light into the waveguide,wherein the first guide surface is arranged to guide light by totalinternal reflection and the second guide surface has a stepped shapecomprising a plurality of extraction facets oriented to reflect inputlight from the light sources, after reflection from the reflective end,through the first guide surface as output light, and intermediateregions between the facets that are arranged to guide light along thewaveguide, the waveguide being arranged to image the light sources sothat the output light is directed into respective optical windows inoutput directions that are distributed laterally in dependence on theinput positions of the light sources; a rear reflector comprising alinear array of reflective facets arranged to reflect light from thelight sources, that is transmitted through the plurality of facets ofthe waveguide, back through the waveguide to exit through the firstguide surface; and a transmissive sheet arranged between the rearreflector and the second guide surface of the waveguide.

The transmissive sheet may comprise plural layers. The plural layers mayinclude a rear protective layer adjacent the rear reflector, the rearprotective layer being made of a material that provides less damage tothe rear reflector than the material of any other layer of the plurallayers. The plural layers may include a front protective layer adjacentthe waveguide, the front protective layer being made of a material thatprovides less damage to the waveguide than the material of any otherlayer of the plural layers. The plural layers include a reinforcinglayer made of a material having a higher stiffness than the materialthan any other layer of the plural layers.

Advantageously damage of the rear reflector and waveguide second guidingsurface may be reduced. Further contrast of Moire between the facets ofthe rear reflector and waveguide respectively may be reduced.

Any of the aspects of the present disclosure may be applied in anycombination.

Embodiments herein may provide an autostereoscopic display that provideswide angle viewing which may allow for directional viewing andconventional 2D compatibility. The wide angle viewing mode may be forobserver tracked autostereoscopic 3D display, observer tracked 2Ddisplay (for example for privacy or power saving applications), for wideviewing angle 2D display or for wide viewing angle stereoscopic 3Ddisplay. Further, embodiments may provide a controlled illuminator forthe purposes of an efficient autostereoscopic display. Such componentscan be used in directional backlights, to provide directional displaysincluding autostereoscopic displays. Additionally, embodiments mayrelate to a directional backlight apparatus and a directional displaywhich may incorporate the directional backlight apparatus. Such anapparatus may be used for autostereoscopic displays, privacy displays,multi-user displays and other directional display applications that mayachieve for example power savings operation and/or high luminanceoperation.

Embodiments herein may provide an autostereoscopic display with largearea and thin structure. Further, as will be described, the opticalvalves of the present disclosure may achieve thin optical componentswith large back working distances. Such components can be used indirectional backlights, to provide directional displays includingautostereoscopic displays. Further, embodiments may provide a controlledilluminator for the purposes of an efficient autostereoscopic display.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiment may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audiovisual systems and electrical and/or opticaldevices. Aspects of the present disclosure may be used with practicallyany apparatus related to optical and electrical devices, opticalsystems, presentation systems or any apparatus that may contain any typeof optical system. Accordingly, embodiments of the present disclosuremay be employed in optical systems, devices used in visual and/oroptical presentations, visual peripherals and so on and in a number ofcomputing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

Directional backlights offer control over the illumination emanatingfrom substantially the entire output surface controlled typicallythrough modulation of independent LED light sources arranged at theinput aperture side of an optical waveguide. Controlling the emittedlight directional distribution can achieve single person viewing for asecurity function, where the display can only be seen by a single viewerfrom a limited range of angles; high electrical efficiency, whereillumination is primarily provided over a small angular directionaldistribution; alternating left and right eye viewing for time sequentialstereoscopic and autostereoscopic display; and low cost.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating a front view of lightpropagation in one embodiment of a directional display device, inaccordance with the present disclosure;

FIG. 1B is a schematic diagram illustrating a side view of lightpropagation in one embodiment of the directional display device of FIG.1A, in accordance with the present disclosure;

FIG. 2A is a schematic diagram illustrating in a top view of lightpropagation in another embodiment of a directional display device, inaccordance with the present disclosure;

FIG. 2B is a schematic diagram illustrating light propagation in a frontview of the directional display device of FIG. 2A, in accordance withthe present disclosure;

FIG. 2C is a schematic diagram illustrating light propagation in a sideview of the directional display device of FIG. 2A, in accordance withthe present disclosure;

FIG. 3 is a schematic diagram illustrating in a side view of adirectional display device, in accordance with the present disclosure;

FIG. 4A is schematic diagram illustrating in a front view, generation ofa viewing window in a directional display device including curved lightextraction features, in accordance with the present disclosure;

FIG. 4B is a schematic diagram illustrating in a front view, generationof a first and a second viewing window in a directional display deviceincluding curved light extraction features, in accordance with thepresent disclosure;

FIG. 5 is a schematic diagram illustrating generation of a first viewingwindow in a directional display device including linear light extractionfeatures, in accordance with the present disclosure;

FIG. 6A is a schematic diagram illustrating one embodiment of thegeneration of a first viewing window in a time multiplexed directionaldisplay device in a first time slot, in accordance with the presentdisclosure;

FIG. 6B is a schematic diagram illustrating another embodiment of thegeneration of a second viewing window in a time multiplexed directionaldisplay device in a second time slot, in accordance with the presentdisclosure;

FIG. 6C is a schematic diagram illustrating another embodiment of thegeneration of a first and a second viewing window in a time multiplexeddirectional display device, in accordance with the present disclosure;

FIG. 7 is a schematic diagram illustrating an observer trackingautostereoscopic directional display device, in accordance with thepresent disclosure;

FIG. 8 is a schematic diagram illustrating a multi-viewer directionaldisplay device, in accordance with the present disclosure;

FIG. 9 is a schematic diagram illustrating a privacy directional displaydevice, in accordance with the present disclosure;

FIG. 10 is a schematic diagram illustrating in side view, the structureof a time multiplexed directional display device, in accordance with thepresent disclosure;

FIG. 11 is a schematic diagram illustrating a directional displayapparatus comprising a directional display device and a control system,in accordance with the present disclosure;

FIG. 12A is a schematic diagram illustrating a perspective view of adirectional display apparatus optical stack comprising a directionalwaveguide with light input at a side that is opposite a reflective side,in accordance with the present disclosure;

FIG. 12B is a schematic diagram illustrating a perspective view of theformation of optical windows by a directional display apparatuscomprising a directional waveguide with light input at a side that isopposite a reflective side, in accordance with the present disclosure;

FIG. 12C is a schematic diagram illustrating a perspective view of adirectional display apparatus optical stack comprising a directionalwaveguide with light input at a side that is adjacent a reflective side,in accordance with the present disclosure;

FIG. 12D is a schematic diagram illustrating a perspective view of theformation of optical windows by a directional display apparatuscomprising a directional waveguide with light input at a side that isadjacent a reflective side, in accordance with the present disclosure;

FIG. 13 is a schematic diagram illustrating a top view of light inputinto a directional waveguide at the input surface, in accordance withthe present disclosure;

FIG. 14, FIG. 15, and FIG. 16 are schematic diagrams illustrating topviews of illumination of a reflective end of a directional waveguide, inaccordance with the present disclosure;

FIG. 17 is a schematic diagram illustrating a perspective view of lightuniformity from a directional waveguide for an off-axis viewing positionfor a wide angle mode of operation, in accordance with the presentdisclosure;

FIG. 18 is a schematic diagram illustrating a perspective view of lightuniformity from a directional waveguide for an off-axis viewing positionfor a privacy mode of operation, in accordance with the presentdisclosure;

FIG. 19 is a schematic diagram illustrating a front view of adirectional waveguide wherein the waveguide further comprises at leastone surface relief feature formed on at least one of the first andsecond guide surfaces in a location adjacent the input surface, inaccordance with the present disclosure;

FIG. 20 is a schematic diagram illustrating a front view of a rearreflector comprising a linear array of reflective facets and furthercomprising alignment holes, in accordance with the present disclosure;

FIG. 21 is a schematic diagram illustrating alignment of the directionalwaveguide of FIG. 19 and the rear reflector of FIG. 20, in accordancewith the present disclosure;

FIG. 22, FIG. 23A, and FIG. 23B are schematic diagrams illustrating aside view of a directional waveguide wherein the waveguide furthercomprises at least one surface relief feature, in accordance with thepresent disclosure;

FIG. 24, FIG. 25A, and FIG. 25B are schematic diagrams illustratingfront views of the input of light into a directional waveguide andbounding regions for surface relief features, in accordance with thepresent disclosure;

FIG. 26A is a schematic diagram illustrating a perspective front view ofoff-axis ray propagation in a directional backlight for a wide viewingmode of operation, in accordance with the present disclosure;

FIG. 26B is a schematic diagram illustrating a perspective front view ofoff-axis ray propagation in a directional backlight for a privacyviewing mode of operation, in accordance with the present disclosure;

FIG. 26C is a schematic diagram illustrating a top view of light inputand light reflection from the input side of the directional waveguide,in accordance with the present disclosure;

FIG. 26D and FIG. 26E are schematic diagrams illustrating side and topviews respectively of a strip arranged to provide reduced reflection oflight rays from an input end, in accordance with the present disclosure;

FIG. 26F is a schematic diagram illustrating a top view of a strip andsupport arranged on the first guiding surface, in accordance with thepresent disclosure;

FIG. 26G is a schematic diagram illustrating a top view of thearrangement of strips on a waveguide, in accordance with the presentdisclosure;

FIG. 26H is a schematic diagram illustrating in side view a directionalbacklight that further comprises a rigid holder portion to which thesupport is attached, in accordance with the present disclosure;

FIG. 26I is a schematic diagram illustrating in side view a directionalbacklight that illustrates that the support may be a rigid holderportion, in accordance with the present disclosure;

FIG. 26J and FIG. 26K are schematic diagrams illustrating in side andtop views a directional backlight comprising diffusive light extractionregions that may be on the first guiding surface and and/or secondguiding surface, in accordance with the present disclosure;

FIG. 26L, FIG. 26M, and FIG. 26N are schematic diagrams illustrating intop view further arrangements of strips and diffusing regions, inaccordance with the present disclosure;

FIG. 26P is a schematic diagram illustrating in side view a directionalbacklight that further comprises a rigid holder portion to which thewaveguide is attached, in accordance with the present disclosure;

FIG. 27 is a schematic diagram illustrating in side view a directionalbacklight comprising further absorptive region on the input side in theregion intermediate the light sources of the array, in accordance withthe present disclosure;

FIG. 28A is a schematic diagram illustrating a perspective front view oflight streaking in a directional waveguide, in accordance with thepresent disclosure;

FIG. 28B is a schematic diagram illustrating in side view thepropagation of light waves in a planar waveguide, in accordance with thepresent disclosure;

FIG. 28C is a schematic diagram illustrating in side view thepropagation of light waves in a waveguide comprising an extractionfeature, in accordance with the present disclosure;

FIG. 28D is a schematic diagram illustrating in side view thepropagation of light rays in a waveguide comprising an extractionfeature, in accordance with the present disclosure;

FIG. 29 is a schematic diagram illustrating a graph of extractedluminance at an extraction feature against angle of light rays withinthe waveguide, in accordance with the present disclosure;

FIG. 30A is a schematic diagram illustrating in side view the mouldingof an extraction feature, in accordance with the present disclosure;

FIG. 30B is a schematic diagram illustrating in side view lightextraction from a light extraction feature comprising a facet dip, inaccordance with the present disclosure;

FIG. 31A is a schematic diagram illustrating in side view preferentialextraction of high angle light in a waveguide, in accordance with thepresent disclosure;

FIG. 31B is a schematic diagram illustrating a graph of extractedluminance at an extraction feature against angle of light rays withinthe waveguide of FIG. 31A, in accordance with the present disclosure;

FIG. 31C and FIG. 31D are schematic diagrams illustrating in side viewprovision of a uniform width strip, in accordance with the presentdisclosure;

FIG. 32 is a schematic diagram illustrating in top view different stripsaligned with a lateral array of light sources, in accordance with thepresent disclosure;

FIG. 33A is a schematic diagram illustrating a side view of adirectional waveguide comprising identification markings, in accordancewith the present disclosure;

FIG. 33B is a schematic diagram illustrating a front view of the inputof light into a directional waveguide and bounding regions for surfacerelief and printed identification features, further comprisingprotrusions from the input surface, in accordance with the presentdisclosure;

FIG. 33C is a schematic diagram illustrating a side view of adirectional waveguide comprising identification markings and furtherprotrusions, in accordance with the present disclosure;

FIG. 33D is a schematic diagram illustrating a top view of a metallizedreflector of a directional waveguide comprising identification markings,in accordance with the present disclosure;

FIG. 34A is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising amale waveguide surface relief alignment feature and a rear reflector, inaccordance with the present disclosure;

FIG. 34B is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide, a rearreflector and a lightbar comprising an array of light emitting elementsand a printed circuit in accordance with the present disclosure;

FIG. 34C is a schematic diagram illustrating a top view of a lightbarcomprising alignment features, in accordance with the presentdisclosure;

FIG. 35A is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide, a rearreflector, a lightbar comprising an array of light emitting elements anda printed circuit and a rear frame, in accordance with the presentdisclosure;

FIG. 35B is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising afemale waveguide surface relief alignment feature and a rear reflectorwith a male surface relief feature, in accordance with the presentdisclosure;

FIG. 35C is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising afemale waveguide surface relief alignment feature and lightbar with amale surface relief feature, in accordance with the present disclosure;

FIG. 36 is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising amale waveguide surface relief alignment feature and an internal framewith a female surface relief alignment feature, in accordance with thepresent disclosure;

FIG. 37 is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising afemale waveguide surface relief alignment feature and an internal framewith a male surface relief alignment feature, in accordance with thepresent disclosure;

FIG. 38 is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector in contact with a directionalwaveguide, in accordance with the present disclosure;

FIG. 39 is a schematic diagram illustrating a side view of a displayapparatus comprising a surface relief feature arranged to provideseparation of rear reflector and waveguide, in accordance with thepresent disclosure;

FIG. 40 is a schematic diagram illustrating in front view a directionalwaveguide and an array of light sources arranged to provide off-axisillumination of voids, in accordance with the present disclosure;

FIG. 41 is a schematic diagram illustrating in front view an array oflightbars for the directional waveguide of FIG. 40, in accordance withthe present disclosure;

FIG. 42 is a schematic diagram illustrating a front view of the input oflight into a directional waveguide and bounding regions for adhesionfeatures, in accordance with the present disclosure;

FIG. 43 is a schematic diagram illustrating a side view of a displayapparatus comprising attachment of a directional waveguide to amechanical arrangement comprising adhesion features, in accordance withthe present disclosure;

FIG. 44, FIG. 45, and FIG. 46 are schematic diagrams illustrating sideviews of a moulding method for a directional waveguide, in accordancewith the present disclosure;

FIG. 47 is a schematic diagram illustrating a side view of a directionalwaveguide comprising printed identification features, in accordance withthe present disclosure;

FIG. 48, FIG. 49, and FIG. 50 are schematic diagrams illustrating sideviews of a moulding method for a directional waveguide comprising mouldinserts, in accordance with the present disclosure;

FIG. 51 is a schematic diagram illustrating a side view of a directionalwaveguide comprising moulded features from the mould inserts of FIGS.48-50, in accordance with the present disclosure;

FIG. 52A is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguide withdamage artifacts, in accordance with the present disclosure;

FIG. 52B is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguideillustrating light propagation due to damage artifacts, in accordancewith the present disclosure;

FIG. 52C is a schematic diagram illustrating a perspective look downview of a display apparatus comprising a rear reflector for adirectional waveguide illustrating light propagation due to damageartifacts, in accordance with the present disclosure;

FIG. 53A, FIG. 53B, and FIG. 53C are schematic diagrams illustratingside views of a display apparatus comprising a rear reflector for adirectional waveguide, further comprising an intermediate layer arrangedbetween the waveguide and rear reflector, in accordance with the presentdisclosure;

FIG. 54 is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguide,wherein the rear reflector further comprises substantially coplanar flatregions, in accordance with the present disclosure;

FIG. 55 is a schematic diagram illustrating a side view of misalignmentof an LED with a valve input side, in accordance with the presentdisclosure;

FIG. 56 is a schematic graph illustrating relative coupling efficiencyof light into a waveguide corresponding horizontal misalignment, inaccordance with the present disclosure;

FIG. 57 is a schematic diagram illustrating a side view of misalignmentof an LED with a valve input side comprising additional reflectiveelements, in accordance with the present disclosure;

FIG. 58 is a schematic graph illustrating relative coupling efficiencyof light into a waveguide further comprising additional reflectiveelements corresponding to vertical and horizontal misalignments, inaccordance with the present disclosure;

FIG. 59A is a schematic diagram illustrating side view of alignment ofan LED array 15 with a waveguide 1 in a first step, in accordance withthe present disclosure;

FIG. 59B is a schematic diagram illustrating side view of alignment ofan LED array 15 with a waveguide 1 in a second step, in accordance withthe present disclosure;

FIG. 60A and FIG. 60B are schematic diagrams illustrating side views ofalignment of an illumination assembly with a mechanical support that isa stop further comprising an adhesive strip, in accordance with thepresent disclosure;

FIG. 61A and FIG. 61B are schematic diagrams illustrating a side view ofalignment of an illumination assembly with a slotted mechanical support,in accordance with the present disclosure;

FIG. 62A and FIG. 62B are schematic diagrams illustrating a side view ofalignment of an illumination assembly with a sprung mechanical support,in accordance with the present disclosure;

FIG. 63A, FIG. 63B, and FIG. 63C are schematic diagrams illustrating aside view of alignment of an illumination assembly with a clippedmechanical support, in accordance with the present disclosure;

FIG. 64A and FIG. 64B are schematic diagrams illustrating top and sideviews respectively of alignment of an LED array with a waveguidecomprising a deformable mechanical support, in accordance with thepresent disclosure; and

FIGS. 65A and FIG. 65B are schematic diagrams illustrating top and sideviews respectively of alignment of side mirror with a waveguidecomprising a deformable mechanical support, in accordance with thepresent disclosure.

DETAILED DESCRIPTION

Time multiplexed autostereoscopic displays can advantageously improvethe spatial resolution of autostereoscopic display by directing lightfrom all of the pixels of a spatial light modulator to a first viewingwindow in a first time slot, and all of the pixels to a second viewingwindow in a second time slot. Thus an observer with eyes arranged toreceive light in first and second viewing windows will see a fullresolution image across the whole of the display over multiple timeslots. Time multiplexed displays can advantageously achieve directionalillumination by directing an illuminator array through a substantiallytransparent time multiplexed spatial light modulator using directionaloptical elements, wherein the directional optical elements substantiallyform an image of the illuminator array in the window plane.

The uniformity of the viewing windows may be advantageously independentof the arrangement of pixels in the spatial light modulator.Advantageously, such displays can provide observer tracking displayswhich have low flicker, with low levels of cross talk for a movingobserver.

To achieve high uniformity in the window plane, it is desirable toprovide an array of illumination elements that have a high spatialuniformity. The illuminator elements of the time sequential illuminationsystem may be provided, for example, by pixels of a spatial lightmodulator with size approximately 100 micrometers in combination with alens array. However, such pixels suffer from similar difficulties as forspatially multiplexed displays. Further, such devices may have lowefficiency and higher cost, requiring additional display components.

High window plane uniformity can be conveniently achieved withmacroscopic illuminators, for example, an array of LEDs in combinationwith homogenizing and diffusing optical elements that are typically ofsize 1 mm or greater. However, the increased size of the illuminatorelements means that the size of the directional optical elementsincreases proportionately. For example, a 16 mm wide illuminator imagedto a 65 mm wide viewing window may require a 200 mm back workingdistance. Thus, the increased thickness of the optical elements canprevent useful application, for example, to mobile displays, or largearea displays.

Addressing the aforementioned shortcomings, optical valves as describedin commonly-owned U.S. Patent Publ. No. 2012/0127573 advantageously canbe arranged in combination with fast switching transmissive spatiallight modulators to achieve time multiplexed autostereoscopicillumination in a thin package while providing high resolution imageswith flicker free observer tracking and low levels of cross talk.Described is a one dimensional array of viewing positions, or windows,that can display different images in a first, typically horizontal,direction, but contain the same images when moving in a second,typically vertical, direction.

Conventional non-imaging display backlights commonly employ opticalwaveguides and have edge illumination from light sources such as LEDs.However, it should be appreciated that there are many fundamentaldifferences in the function, design, structure, and operation betweensuch conventional non-imaging display backlights and the imagingdirectional backlights discussed in the present disclosure.

Generally, for example, in accordance with the present disclosure,imaging directional backlights are arranged to direct the illuminationfrom multiple light sources through a display panel to respectivemultiple viewing windows in at least one axis. Each viewing window issubstantially formed as an image in at least one axis of a light sourceby the imaging system of the imaging directional backlight. An imagingsystem may be formed between multiple light sources and the respectivewindow images. In this manner, the light from each of the multiple lightsources is substantially not visible for an observer's eye outside ofthe respective viewing window.

In contradistinction, conventional non-imaging backlights or lightguiding plates (LGPs) are used for illumination of 2D displays. See,e.g., Kalil Käläntär et al., Backlight Unit With Double Surface LightEmission, J. Soc. Inf. Display, Vol. 12, Issue 4, pp. 379-387 (December2004). Non-imaging backlights are typically arranged to direct theillumination from multiple light sources through a display panel into asubstantially common viewing zone for each of the multiple light sourcesto achieve wide viewing angle and high display uniformity. Thusnon-imaging backlights do not form viewing windows. In this manner, thelight from each of the multiple light sources may be visible for anobserver's eye at substantially all positions across the viewing zone.Such conventional non-imaging backlights may have some directionality,for example, to increase screen gain compared to Lambertianillumination, which may be provided by brightness enhancement films suchas BEF™ from 3M. However, such directionality may be substantially thesame for each of the respective light sources. Thus, for these reasonsand others that should be apparent to persons of ordinary skill,conventional non-imaging backlights are different to imaging directionalbacklights. Edge lit non-imaging backlight illumination structures maybe used in liquid crystal display systems such as those seen in 2DLaptops, Monitors and TVs. Light propagates from the edge of a lossywaveguide which may include sparse features; typically localindentations in the surface of the guide which cause light to be lostregardless of the propagation direction of the light.

As used herein, an optical valve is an optical structure that may be atype of light guiding structure or device referred to as, for example, alight valve, an optical valve directional backlight, and a valvedirectional backlight (“v-DBL”). In the present disclosure, opticalvalve is different to a spatial light modulator (even though spatiallight modulators may be sometimes generally referred to as a “lightvalve” in the art). One example of an imaging directional backlight isan optical valve that may employ a folded optical system. Light maypropagate substantially without loss in one direction through theoptical valve, may be incident on an imaging reflector, and maycounter-propagate such that the light may be extracted by reflection offtilted light extraction features, and directed to viewing windows asdescribed in U.S. Patent Publ. No. 2012/0127573, which is hereinincorporated by reference in its entirety.

Additionally, as used herein, a stepped waveguide imaging directionalbacklight may be at least one of an optical valve. A stepped waveguideis a waveguide for an imaging directional backlight comprising awaveguide for guiding light, further comprising: a first light guidingsurface; and a second light guiding surface, opposite the first lightguiding surface, further comprising a plurality of light guidingfeatures interspersed with a plurality of extraction features arrangedas steps.

In operation, light may propagate within an exemplary optical valve in afirst direction from an input surface to a reflective side and may betransmitted substantially without loss. Light may be reflected at thereflective side and propagates in a second direction substantiallyopposite the first direction. As the light propagates in the seconddirection, the light may be incident on light extraction features, whichare operable to redirect the light outside the optical valve. Stateddifferently, the optical valve generally allows light to propagate inthe first direction and may allow light to be extracted whilepropagating in the second direction.

The optical valve may achieve time sequential directional illuminationof large display areas. Additionally, optical elements may be employedthat are thinner than the back working distance of the optical elementsto direct light from macroscopic illuminators to a window plane. Suchdisplays may use an array of light extraction features arranged toextract light counter propagating in a substantially parallel waveguide.

Thin imaging directional backlight implementations for use with LCDshave been proposed and demonstrated by 3M, for example U.S. Pat. No.7,528,893; by Microsoft, for example U.S. Pat. No. 7,970,246 which maybe referred to herein as a “wedge type directional backlight;” by RealD,for example U.S. Patent Publ. No. 2012/0127573 which may be referred toherein as an “optical valve” or “optical valve directional backlight,”all of which are herein incorporated by reference in their entirety.

The present disclosure provides stepped waveguide imaging directionalbacklights in which light may reflect back and forth between theinternal faces of, for example, a stepped waveguide which may include afirst side and a first set of features. As the light travels along thelength of the stepped waveguide, the light may not substantially changeangle of incidence with respect to the first side and first set ofsurfaces and so may not reach the critical angle of the medium at theseinternal faces. Light extraction may be advantageously achieved by asecond set of surfaces (the step “risers”) that are inclined to thefirst set of surfaces (the step “treads”). Note that the second set ofsurfaces may not be part of the light guiding operation of the steppedwaveguide, but may be arranged to provide light extraction from thestructure. By contrast, a wedge type imaging directional backlight mayallow light to guide within a wedge profiled waveguide having continuousinternal surfaces. The optical valve is thus not a wedge type imagingdirectional backlight.

FIG. 1A is a schematic diagram illustrating a front view of lightpropagation in one embodiment of a directional display device, and FIG.1B is a schematic diagram illustrating a side view of light propagationin the directional display device of FIG. 1A.

FIG. 1A illustrates a front view in the xy plane of a directionalbacklight of a directional display device, and includes an illuminatorarray 15 which may be used to illuminate a stepped waveguide 1.Illuminator array 15 includes illuminator elements 15 a throughilluminator element 15 n (where n is an integer greater than one). Inone example, the stepped waveguide 1 of FIG. 1A may be a stepped,display sized waveguide 1. Illumination elements 15 a through 15 n arelight sources that may be light emitting diodes (LEDs). Although LEDsare discussed herein as illuminator elements 15 a-15 n, other lightsources may be used such as, but not limited to, diode sources,semiconductor sources, laser sources, local field emission sources,organic emitter arrays, and so forth. Additionally, FIG. 1B illustratesa side view in the xz plane, and includes illuminator array 15, SLM 48,extraction features 12, guiding features 10, and stepped waveguide 1,arranged as shown. The side view provided in FIG. 1B is an alternativeview of the front view shown in FIG. 1A. Accordingly, the illuminatorarray 15 of FIGS. 1A and 1B corresponds to one another and the steppedwaveguide 1 of FIGS. 1A and 1B may correspond to one another.

Further, in FIG. 1B, the stepped waveguide 1 may have an input end 2that is thin and a reflective end 4 that is thick. Thus the waveguide 1extends between the input end 2 that receives input light and thereflective end 4 that reflects the input light back through thewaveguide 1. The length of the input end 2 in a lateral direction acrossthe waveguide is greater than the height of the input end 2. Theilluminator elements 15 a-15 n are disposed at different input positionsin a lateral direction across the input end 2.

The waveguide 1 has first and second, opposed guide surfaces extendingbetween the input end 2 and the reflective end 4 for guiding lightforwards and back along the waveguide 1. The second guide surface has aplurality of light extraction features 12 facing the reflective end 4and arranged to reflect at least some of the light guided back throughthe waveguide 1 from the reflective end from different input positionsacross the input end in different directions through the first guidesurface that are dependent on the input position.

In this example, the light extraction features 12 are reflective facets,although other reflective features could be used. The light extractionfeatures 12 do not guide light through the waveguide, whereas theintermediate regions of the second guide surface intermediate the lightextraction features 12 guide light without extracting it. Those regionsof the second guide surface are planar and may extend parallel to thefirst guide surface, or at a relatively low inclination. The lightextraction features 12 extend laterally to those regions so that thesecond guide surface has a stepped shape which may include the lightextraction features 12 and intermediate regions. The light extractionfeatures 12 are oriented to reflect light from the light sources, afterreflection from the reflective end 4, through the first guide surface.

The light extraction features 12 are arranged to direct input light fromdifferent input positions in the lateral direction across the input endin different directions relative to the first guide surface that aredependent on the input position. As the illumination elements 15 a-15 nare arranged at different input positions, the light from respectiveillumination elements 15 a-15 n is reflected in those differentdirections. In this manner, each of the illumination elements 15 a-15 ndirects light into a respective optical window in output directionsdistributed in the lateral direction in dependence on the inputpositions. The lateral direction across the input end 2 in which theinput positions are distributed corresponds with regard to the outputlight to a lateral direction to the normal to the first guide surface.The lateral directions as defined at the input end 2 and with regard tothe output light remain parallel in this embodiment where thedeflections at the reflective end 4 and the first guide surface aregenerally orthogonal to the lateral direction. Under the control of acontrol system, the illuminator elements 15 a-15 n may be selectivelyoperated to direct light into a selectable optical window. The opticalwindows may be used individually or in groups as viewing windows.

The SLM 48 extends across the waveguide and modulates the light outputtherefrom. Although the SLM 48 may a liquid crystal display (LCD), thisis merely by way of example and other spatial light modulators ordisplays may be used including LCOS, DLP devices, and so forth, as thisilluminator may work in reflection. In this example, the SLM 48 isdisposed across the first guide surface of the waveguide and modulatesthe light output through the first guide surface after reflection fromthe light extraction features 12.

The operation of a directional display device that may provide a onedimensional array of viewing windows is illustrated in front view inFIG. 1A, with its side profile shown in FIG. 1B. In operation, in FIGS.1A and 1B, light may be emitted from an illuminator array 15, such as anarray of illuminator elements 15 a through 15 n, located at differentpositions, y, along the surface of thin end side 2, x=0, of the steppedwaveguide 1. The light may propagate along +x in a first direction,within the stepped waveguide 1, while at the same time, the light mayfan out in the xy plane and upon reaching the far curved end side 4, maysubstantially or entirely fill the curved end side 4. While propagating,the light may spread out to a set of angles in the xz plane up to, butnot exceeding the critical angle of the guide material. The extractionfeatures 12 that link the guiding features 10 of the bottom side of thestepped waveguide 1 may have a tilt angle greater than the criticalangle and hence may be missed by substantially all light propagatingalong +x in the first direction, ensuring the substantially losslessforward propagation.

Continuing the discussion of FIGS. 1A and 1B, the curved end side 4 ofthe stepped waveguide 1 may be made reflective, typically by beingcoated with a reflective material such as, for example, silver, althoughother reflective techniques may be employed. Light may therefore beredirected in a second direction, back down the guide in the directionof −x and may be substantially collimated in the xy or display plane.The angular spread may be substantially preserved in the xz plane aboutthe principal propagation direction, which may allow light to hit theriser edges and reflect out of the guide. In an embodiment withapproximately 45 degree tilted extraction features 12, light may beeffectively directed approximately normal to the xy display plane withthe xz angular spread substantially maintained relative to thepropagation direction. This angular spread may be increased when lightexits the stepped waveguide 1 through refraction, but may be decreasedsomewhat dependent on the reflective properties of the extractionfeatures 12.

In some embodiments with uncoated extraction features 12, reflection maybe reduced when total internal reflection (TIR) fails, squeezing the xzangular profile and shifting off normal. However, in other embodimentshaving silver coated or metallized extraction features, the increasedangular spread and central normal direction may be preserved. Continuingthe description of the embodiment with silver coated extractionfeatures, in the xz plane, light may exit the stepped waveguide 1approximately collimated and may be directed off normal in proportion tothe y-position of the respective illuminator element 15 a-15 n inilluminator array 15 from the input edge center. Having independentilluminator elements 15 a-15 n along the input edge 2 then enables lightto exit from the entire first light directing side 6 and propagate atdifferent external angles, as illustrated in FIG. 1A.

Illuminating a spatial light modulator (SLM) 48 such as a fast liquidcrystal display (LCD) panel with such a device may achieveautostereoscopic 3D as shown in top view or yz-plane viewed from theilluminator array 15 end in FIG. 2A, front view in FIG. 2B and side viewin FIG. 2C. FIG. 2A is a schematic diagram illustrating in a top view,propagation of light in a directional display device, FIG. 2B is aschematic diagram illustrating in a front view, propagation of light ina directional display device, and FIG. 2C is a schematic diagramillustrating in side view propagation of light in a directional displaydevice. As illustrated in FIGS. 2A, 2B, and 2C, a stepped waveguide 1may be located behind a fast (e.g., greater than 100 Hz) LCD panel SLM48 that displays sequential right and left eye images. Insynchronization, specific illuminator elements 15 a through 15 n ofilluminator array 15 (where n is an integer greater than one) may beselectively turned on and off, providing illuminating light that entersright and left eyes substantially independently by virtue of thesystem's directionality. In the simplest case, sets of illuminatorelements of illuminator array 15 are turned on together, providing a onedimensional viewing window 26 or an optical pupil with limited width inthe horizontal direction, but extended in the vertical direction, inwhich both eyes horizontally separated may view a left eye image, andanother viewing window 44 in which a right eye image may primarily beviewed by both eyes, and a central position in which both the eyes mayview different images. In this way, 3D may be viewed when the head of aviewer is approximately centrally aligned. Movement to the side awayfrom the central position may result in the scene collapsing onto a 2Dimage.

The reflective end 4 may have positive optical power in the lateraldirection across the waveguide. 14. In other words, the reflective endmay have positive optical power in a direction extending between sidesof the waveguide that extend between the first and second guide surfacesand between the input end and the reflective end. The light extractionfeatures 12 may have positive optical power in a direction between sidesof the waveguide that extend between the first and second guide surfaces6,8 and between the input end 2 and the reflective end.

The waveguide 1 may further comprising a reflective end 4 for reflectinginput light from the light sources back along the waveguide 1, thesecond guide surface 8 being arranged to deflect the reflected inputlight through the first guide surface 6 as output light, and thewaveguide 1 being arranged to image the light sources 15 a-n so that theoutput light from the light sources is directed into respective opticalwindows 26 a-n in output directions that are distributed laterally independence on the input positions of the light sources.

In embodiments in which typically the reflective end 4 has positiveoptical power, the optical axis may be defined with reference to theshape of the reflective end 4, for example being a line that passesthrough the center of curvature of the reflective end 4 and coincideswith the axis of reflective symmetry of the end 4 about the x-axis. Inthe case that the reflecting surface 4 is flat, the optical axis may besimilarly defined with respect to other components having optical power,for example the light extraction features 12 if they are curved, or theFresnel lens 62 described below. The optical axis 238 is typicallycoincident with the mechanical axis of the waveguide 1.In the presentembodiments that typically comprise a substantially cylindricalreflecting surface at end 4, the optical axis 238 is a line that passesthrough the center of curvature of the surface at end 4 and coincideswith the axis of reflective symmetry of the side 4 about the x-axis. Theoptical axis 238 is typically coincident with the mechanical axis of thewaveguide 1. The cylindrical reflecting surface at end 4 may typicallycomprise a spherical profile to optimize performance for on-axis andoff-axis viewing positions. Other profiles may be used.

FIG. 3 is a schematic diagram illustrating in side view a directionaldisplay device. Further, FIG. 3 illustrates additional detail of a sideview of the operation of a stepped waveguide 1, which may be atransparent material. The stepped waveguide 1 may include an illuminatorinput side 2, a reflective side 4, a first light directing side 6 whichmay be substantially planar, and a second light directing side 8 whichincludes guiding features 10 and light extraction features 12. Inoperation, light rays 16 from an illuminator element 15 c of anilluminator array 15 (not shown in FIG. 3), that may be an addressablearray of LEDs for example, may be guided in the stepped waveguide 1 bymeans of total internal reflection by the first light directing side 6and total internal reflection by the guiding feature 10, to thereflective side 4, which may be a mirrored surface. Although reflectiveside 4 may be a mirrored surface and may reflect light, it may in someembodiments also be possible for light to pass through reflective side4.

Continuing the discussion of FIG. 3, light ray 18 reflected by thereflective side 4 may be further guided in the stepped waveguide 1 bytotal internal reflection at the reflective side 4 and may be reflectedby extraction features 12. Light rays 18 that are incident on extractionfeatures 12 may be substantially deflected away from guiding modes ofthe stepped waveguide 1 and may be directed, as shown by ray 20, throughthe side 6 to an optical pupil that may form a viewing window 26 of anautostereoscopic display. The width of the viewing window 26 may bedetermined by at least the size of the illuminator, output designdistance and optical power in the side 4 and extraction features 12. Theheight of the viewing window may be primarily determined by thereflection cone angle of the extraction features 12 and the illuminationcone angle input at the input side 2. Thus each viewing window 26represents a range of separate output directions with respect to thesurface normal direction of the spatial light modulator 48 thatintersect with a plane at the nominal viewing distance.

FIG. 4A is a schematic diagram illustrating in front view a directionaldisplay device which may be illuminated by a first illuminator elementand including curved light extraction features. Further, FIG. 4A showsin front view further guiding of light rays from illuminator element 15c of illuminator array 15, in the stepped waveguide 1. Each of theoutput rays are directed towards the same viewing window 26 from therespective illuminator 14. Thus light ray 30 may intersect the ray 20 inthe window 26, or may have a different height in the window as shown byray 32. Additionally, in various embodiments, sides 22, 24 of thewaveguide 1 may be transparent, mirrored, or blackened surfaces.Continuing the discussion of FIG. 4A, light extraction features 12 maybe elongate, and the orientation of light extraction features 12 in afirst region 34 of the light directing side 8 (light directing side 8shown in FIG. 3, but not shown in FIG. 4A) may be different to theorientation of light extraction features 12 in a second region 36 of thelight directing side 8.

FIG. 4B is a schematic diagram illustrating in front view an opticalvalve which may illuminated by a second illuminator element. Further,FIG. 4B shows the light rays 40, 42 from a second illuminator element 15h of the illuminator array 15. The curvature of the reflective end onthe side 4 and the light extraction features 12 cooperatively produce asecond viewing window 44 laterally separated from the viewing window 26with light rays from the illuminator element 15 h.

Advantageously, the arrangement illustrated in FIG. 4B may provide areal image of the illuminator element 15 c at a viewing window 26 inwhich the real image may be formed by cooperation of optical power inreflective side 4 and optical power which may arise from differentorientations of elongate light extraction features 12 between regions 34and 36, as shown in FIG. 4A. The arrangement of FIG. 4B may achieveimproved aberrations of the imaging of illuminator element 15 c tolateral positions in viewing window 26. Improved aberrations may achievean extended viewing freedom for an autostereoscopic display whileachieving low cross talk levels.

FIG. 5 is a schematic diagram illustrating in front view an embodimentof a directional display device having substantially linear lightextraction features. Further, FIG. 5 shows a similar arrangement ofcomponents to FIG. 1 (with corresponding elements being similar), withone of the differences being that the light extraction features 12 aresubstantially linear and parallel to each other. Advantageously, such anarrangement may provide substantially uniform illumination across adisplay surface and may be more convenient to manufacture than thecurved extraction features of FIG. 4A and FIG. 4B.

FIG. 6A is a schematic diagram illustrating one embodiment of thegeneration of a first viewing window in a time multiplexed imagingdirectional display device in a first time slot, FIG. 6B is a schematicdiagram illustrating another embodiment of the generation of a secondviewing window in a time multiplexed imaging directional backlightapparatus in a second time slot, and FIG. 6C is a schematic diagramillustrating another embodiment of the generation of a first and asecond viewing window in a time multiplexed imaging directional displaydevice. Further, FIG. 6A shows schematically the generation ofillumination window 26 from stepped waveguide 1. Illuminator elementgroup 31 in illuminator array 15 may provide a light cone 17 directedtowards a viewing window 26. FIG. 6B shows schematically the generationof illumination window 44. Illuminator element group 33 in illuminatorarray 15 may provide a light cone 19 directed towards viewing window 44.In cooperation with a time multiplexed display, windows 26 and 44 may beprovided in sequence as shown in FIG. 6C. If the image on a spatiallight modulator 48 (not shown in FIGS. 6A, 6B, 6C) is adjusted incorrespondence with the light direction output, then an autostereoscopicimage may be achieved for a suitably placed viewer. Similar operationcan be achieved with all the directional backlights described herein.Note that illuminator element groups 31, 33 each include one or moreillumination elements from illumination elements 15 a to 15 n, where nis an integer greater than one.

FIG. 7 is a schematic diagram illustrating one embodiment of an observertracking autostereoscopic directional display device. As shown in FIG.7, selectively turning on and off illuminator elements 15 a to 15 nalong axis 29 provides for directional control of viewing windows. Thehead 45 position may be monitored with a camera, motion sensor, motiondetector, or any other appropriate optical, mechanical or electricalmeans, and the appropriate illuminator elements of illuminator array 15may be turned on and off to provide substantially independent images toeach eye irrespective of the head 45 position. The head tracking system(or a second head tracking system) may provide monitoring of more thanone head 45, 47 (head 47 not shown in FIG. 7) and may supply the sameleft and right eye images to each viewers' left and right eyes providing3D to all viewers. Again similar operation can be achieved with all thedirectional backlights described herein.

FIG. 8 is a schematic diagram illustrating one embodiment of amulti-viewer directional display device as an example including animaging directional backlight. As shown in FIG. 8, at least two 2Dimages may be directed towards a pair of viewers 45, 47 so that eachviewer may watch a different image on the spatial light modulator 48.The two 2D images of FIG. 8 may be generated in a similar manner asdescribed with respect to FIG. 7 in that the two images would bedisplayed in sequence and in synchronization with sources whose light isdirected toward the two viewers. One image is presented on the spatiallight modulator 48 in a first phase, and a second image is presented onthe spatial light modulator 48 in a second phase different from thefirst phase. In correspondence with the first and second phases, theoutput illumination is adjusted to provide first and second viewingwindows 26, 44 respectively. An observer with both eyes in window 26will perceive a first image while an observer with both eyes in window44 will perceive a second image.

FIG. 9 is a schematic diagram illustrating a privacy directional displaydevice which includes an imaging directional backlight. 2D displaysystems may also utilize directional backlighting for security andefficiency purposes in which light may be primarily directed at the eyesof a first viewer 45 as shown in FIG. 9. Further, as illustrated in FIG.9, although first viewer 45 may be able to view an image on device 50,light is not directed towards second viewer 47. Thus second viewer 47 isprevented from viewing an image on device 50. Each of the embodiments ofthe present disclosure may advantageously provide autostereoscopic, dualimage or privacy display functions.

FIG. 10 is a schematic diagram illustrating in side view the structureof a time multiplexed directional display device as an example includingan imaging directional backlight. Further, FIG. 10 shows in side view anautostereoscopic directional display device, which may include thestepped waveguide 1 and a Fresnel lens 62 arranged to provide theviewing window 26 in a window plane 106 at a nominal viewing distancefrom the spatial light modulator for a substantially collimated outputacross the stepped waveguide 1 output surface. A vertical diffuser 68may be arranged to extend the height of the window 26 further. The lightmay then be imaged through the spatial light modulator 48. Theilluminator array 15 may include light emitting diodes (LEDs) that may,for example, be phosphor converted blue LEDs, or may be separate RGBLEDs. Alternatively, the illuminator elements in illuminator array 15may include a uniform light source and spatial light modulator arrangedto provide separate illumination regions. Alternatively the illuminatorelements may include laser light source(s). The laser output may bedirected onto a diffuser by means of scanning, for example, using agalvo or MEMS scanner. In one example, laser light may thus be used toprovide the appropriate illuminator elements in illuminator array 15 toprovide a substantially uniform light source with the appropriate outputangle, and further to provide reduction in speckle. Alternatively, theilluminator array 15 may be an array of laser light emitting elements.Additionally in one example, the diffuser may be a wavelength convertingphosphor, so that illumination may be at a different wavelength to thevisible output light.

A further wedge type directional backlight is generally discussed byU.S. Pat. No. 7,660,047 which is herein incorporated by reference in itsentirety. The wedge type directional backlight and optical valve furtherprocess light beams in different ways. In the wedge type waveguide,light input at an appropriate angle will output at a defined position ona major surface, but light rays will exit at substantially the sameangle and substantially parallel to the major surface. By comparison,light input to a stepped waveguide of an optical valve at a certainangle may output from points across the first side, with output angledetermined by input angle. Advantageously, the stepped waveguide of theoptical valve may not require further light re-direction films toextract light towards an observer and angular non-uniformities of inputmay not provide non-uniformities across the display surface.

There will now be described some waveguides, directional backlights anddirectional display devices that are based on and incorporate thestructures of FIGS. 1 to 10 above. Except for the modifications and/oradditional features which will now be described, the above descriptionapplies equally to the following waveguides, directional backlights anddisplay devices, but for brevity will not be repeated. The waveguidesdescribed below may be incorporated into a directional backlight or adirectional display device as described above. Similarly, thedirectional backlights described below may be incorporated into adirectional display device as described above.

FIG. 11 is a schematic diagram illustrating a directional displayapparatus comprising a directional display device and a control system.The arrangement and operation of the control system will now bedescribed and may be applied, with changes as necessary, to each of thedisplay devices disclosed herein. The directional backlight comprises awaveguide 1 and an array 15 of illumination elements 15 a-15 n arrangedas described above. The control system is arranged to selectivelyoperate the illumination elements 15 a-15 n to direct light intoselectable viewing windows.

The reflective end 4 converges the reflected light. Fresnel lens 62 maybe arranged to cooperate with reflective end 4 to achieve viewingwindows at a viewing plane. Transmissive spatial light modulator 48 maybe arranged to receive the light from the directional backlight. Theimage displayed on the SLM 48 may be presented in synchronization withthe illumination of the light sources of the array 15.

The control system may comprise a sensor system arranged to detect theposition of the observer 99 relative to the display device 100. Thesensor system comprises a position sensor 406, such as a camera arrangedto determine the position of an observer 408; and a head positionmeasurement system 404 that may for example comprise a computer visionimage processing system. The position sensor 406 may comprise knownsensors including those comprising cameras and image processing unitsarranged to detect the position of observer faces. Position sensor 406may further comprise a stereo sensor arranged to improve the measure oflongitudinal position compared to a monoscopic camera. Alternativelyposition sensor 406 may comprise measurement of eye spacing to give ameasure of required placement of respective arrays of viewing windowsfrom tiles of the directional display.

The control system may further comprise an illumination controller andan image controller 403 that are both supplied with the detectedposition of the observer supplied from the head position measurementsystem 404.

The illumination controller comprises an LED controller 402 arranged todetermine which light sources of array 15 should be switched to directlight to respective eyes of observer 408 in cooperation with waveguide1; and an LED driver 400 arranged to control the operation of lightsources of light source array 15 by means of drive lines 407. Theillumination controller 74 selects the illuminator elements 15 to beoperated in dependence on the position of the observer detected by thehead position measurement system 72, so that the viewing windows 26 intowhich light is directed are in positions corresponding to the left andright eyes of the observer 99. In this manner, the lateral outputdirectionality of the waveguide 1 corresponds with the observerposition.

The image controller 403 is arranged to control the SLM 48 to displayimages. To provide an autostereoscopic display, the image controller 403and the illumination controller may operate as follows. The imagecontroller 403 controls the SLM 48 to display temporally multiplexedleft and right eye images and the LED controller 402 operates the lightsources 15 to direct light into viewing windows in positionscorresponding to the left and right eyes of an observer synchronouslywith the display of left and right eye images. In this manner, anautostereoscopic effect is achieved using a time division multiplexingtechnique. In one example, a single viewing window may be illuminated byoperation of light source 409 (which may comprise one or more LEDs) bymeans of drive line 410 wherein other drive lines are not driven asdescribed elsewhere.

The head position measurement system 404 detects the position of anobserver relative to the display device 100. The LED controller 402selects the light sources 15 to be operated in dependence on theposition of the observer detected by the head position measurementsystem 404, so that the viewing windows into which light is directed arein positions corresponding to the left and right eyes of the observer.In this manner, the output directionality of the waveguide 1 may beachieved to correspond with the viewer position so that a first imagemay be directed to the observer's right eye in a first phase anddirected to the observer's left eye in a second phase.

Thus a directional display apparatus may comprise a directional displaydevice and a control system arranged to control the light sources 15a-n.

FIG. 12A is a schematic diagram illustrating a perspective view of adirectional display apparatus optical stack comprising a directionalwaveguide with light input at a side that is opposite a reflective side.

Reflective end 4 may be provided by a Fresnel mirror. Further taperregion 204 may be arranged at the input to the waveguide 1 to increaseinput coupling efficiency from the light sources 15 a-15 n of the arrayof illuminator elements 15 and to increase illumination uniformity.Shading layer 206 with aperture 203 may be arranged to hide lightscattering regions at the edge of the waveguide 1. Rear reflector 300may comprise facets 302 that are curved and arranged to provide viewingwindows from groups of optical windows provided by imaging light sourcesof the array 15 to the window plane. An optical stack 208 may comprisereflective polarizers, retarder layers and diffusers. Rear reflectors300 and optical stack 208 are described further in U.S. Patent Publ. No.2014/0240828, filed Feb. 21, 2014, entitled “Directional backlight”(Attorney Ref. No. 355001) incorporated herein by reference in itsentirety.

Spatial light modulator 48 may comprise a liquid crystal display thatmay comprise an input polarizer 210, TFT glass substrate 212, liquidcrystal layer 214, color filter glass substrate 216 and output polarizer218. Red pixels 220, green pixels 222 and blue pixels 224 may bearranged in an array at the liquid crystal layer 214. White, yellow,additional green or other color pixels (not shown) may be furtherarranged in the liquid crystal layer to increase transmissionefficiency, color gamut or perceived image resolution.

FIG. 12B is a schematic diagram illustrating a perspective view of theformation of optical windows by a directional display apparatuscomprising a directional waveguide with light input at a side that isopposite a reflective side. The input surface 2 may thus be an end ofthe waveguide 1 may be opposite to the reflective end.

FIG. 12C is a schematic diagram illustrating a perspective view of adirectional display apparatus optical stack comprising a directionalwaveguide with light input at a side that is adjacent a reflective sideas described elsewhere in U.S. patent application Ser. No. 15/165,960,entitled “Wide angle imaging directional backlights,” filed May 26, 2016(Attorney Ref. No. 384001) and incorporated by reference herein in itsentirety. Waveguide 301 comprises input sides 322, 324 with alignedlight sources 317 a-n and 319 a-n on respective sides. End 302 oppositereflective end 304 may be arranged to be absorbing or reflective toprovide low levels of cross talk or increased efficiency respectively.

FIG. 12D is a schematic diagram illustrating a perspective view of theformation of optical windows by a directional display apparatuscomprising a directional waveguide with light input at a side that isadjacent a reflective side. Light sources 317 a-n and 319 a-n at inputfacets 321 are arranged to provide optical windows 27 a-n and 29 a-nrespectively about an axis 197. Fresnel mirror 304 is arranged withfirst and second optical axes 287, 289. The input surface may thus be aside surface 322 of the waveguide 301 extending away from the reflectiveend 304.

A directional backlight thus comprises a first guide surface 6 arrangedto guide light by total internal reflection and the second guide surface8 comprising a plurality of light extraction features 12 oriented todirect light guided along the waveguide 1, 301 in directions allowingexit through the first guide surface 6 as the output light andintermediate regions 10 between the light extraction features 12 thatare arranged to guide light along the waveguide 1, 301.

Considering the arrangements of FIGS. 12A-D, the second guide surface 6may have a stepped shape in which said light extraction features 12 arefacets between the intermediate regions 10. The light extractionfeatures 12 may have positive optical power in a direction between theside surfaces 22, 24 or 322, 324 of the waveguide 1, 301 that extendbetween the first and second guide surfaces 6,8. The reflective end 4,304 may have positive optical power in a direction extending between thesides 22,24 or 322, 324 of the reflective end 4, 304 that extend betweenthe first and second guide surfaces 6,8.

Thus all sides 2, 4, 6, 8, 22, 24 provide reflections to achieve uniformillumination and low cross talk in privacy mode of operation. Iffeatures are applied to many areas of the surface then non-uniformitiesmay be provided due to the spatial location of the waveguide extractionloss at the features.

It may be desirable to provide high image uniformity by means ofillumination of the reflective end 4, 304.

FIG. 13 is a schematic diagram illustrating a top view of light inputinto a directional waveguide at the input surface. Considering thearrangement of FIG. 12A, light cone 322 is provided by light source 15 bby illumination through input facets 326, separated by draft facets 328on side 2. Facet 326 may further comprise microstructures to vary theillumination angular profile 324 of input light. Facets 326 may betilted to direct light to the reflective end 4.

FIGS. 14-16 are schematic diagrams illustrating top views ofillumination of a reflective end of a directional waveguide. Lightsource 15 is arranged to illuminate reflective end 4 from input surface2. Rays 502,504 describe the limit of rays that are provided by lightsource and illuminate end 4. As shown in FIG. 15 regions 500 are regionsof the input surface through which input light from light sources 15 a-nto reflective end 4 does not propagate. Similarly for the arrangement ofFIG. 16 regions 500 are regions of the input surface through which inputlight from light sources 317 a-n and 319 a-n to reflective end 304 doesnot propagate.

FIG. 17 is a schematic diagram illustrating a perspective view of lightuniformity from a directional waveguide for an off-axis viewing positionfor a wide angle mode of operation. Image 762 is illuminated by uniformlight 766 and void 768. Void 768 can be filled by off-axis illuminationas described in U.S. Patent Publ. No. 2016/0299281, entitled “Wide angleimaging directional backlights” (Attorney Ref. No. 379001), which isincorporated by reference herein in its entirety. It may be desirable tomaintain the uniformity of region 766.

FIG. 18 is a schematic diagram illustrating a perspective view of lightuniformity from a directional waveguide for an off-axis viewing positionfor a privacy mode of operation where only on-axis light sources areprovided. High reflectivity regions on the input surface may providelight streaks such as streak 767. It may be desirable to reduce theintensity of streak 767, advantageously improving privacy performance.

The arrangements of FIGS. 12A and 12C desirable require alignment ofmultiple components such as waveguides, rear reflectors, optical stacksand LCD components. It may be desirable to reduce the cost andcomplexity of assembly of such components. Further such alignmentmethods should provide high image uniformity. It would further bedesirable for alignment features to compensate for differential thermalexpansion of components in the display apparatus.

Thus it would be further desirable to provide a directional backlightthat reduces the amount of stray light seen in Privacy mode ofoperation.

FIG. 19 is a schematic diagram illustrating a front view of adirectional waveguide wherein the waveguide further comprises at leastone surface relief feature formed on at least one of the first andsecond guide surfaces in a location adjacent the input surface.Waveguide 1 is formed with a number of alignment features 510 in regionthat is close to input surface 2 as will be described. Sections A-A′ andB-B′ will be described elsewhere herein.

FIG. 20 is a schematic diagram illustrating a front view of a rearreflector comprising a linear array of reflective facets and furthercomprising alignment features 512. Features 512 may be circular,elongate slot or other shapes.

FIG. 21 is a schematic diagram illustrating alignment of the directionalwaveguide of FIG. 19 and the rear reflector of FIG. 20. Thus features510, 512 may be aligned during an assembly step. The features 510 may bepins that align with features 512 that may be holes in the rearreflector.

Advantageously the waveguide 1 and rear reflector 300 may be aligned.Further differential thermal expansion in the lateral direction(y-axis), while maintaining alignment in the orthogonal direction(x-axis).

FIGS. 22-23B are schematic diagrams illustrating a side view of adirectional waveguide wherein the waveguide further comprises at leastone surface relief feature. Such side view may be a cross section ofsection B-B′ in FIG. 19 for example. Thus a directional backlight maycomprise: a waveguide 1 comprising first and second, opposed guidesurfaces 6,8 for guiding light along the waveguide 1 and an inputsurface 2 extending between the first and second guide surfaces 6,8; andan array of light sources 15 a-n arranged at different input positionsalong the input surface 2 of the waveguide 1 and arranged to input lightinto the waveguide 1, the light sources 15 a-n having light emittingregions that are spaced apart, the waveguide 1 further comprising areflective end 4 for reflecting input light from the light sources 15a-n back along the waveguide 1, the second guide surface 8 beingarranged to deflect the reflected input light through the first guidesurface 6 as output light, and the directional backlight being arrangedto direct the output light into optical windows 26 in output directionsthat are distributed laterally in dependence on the input positionsalong the input surface 2 of the light sources that inputted the inputlight, wherein the waveguide 1 further comprises at least one surfacerelief feature 510 formed on at least one of the first and second guidesurfaces 6,8.

Further the waveguide 1 may comprise a reflective end 2 for reflectinginput light from the light sources back along the waveguide 1, thesecond guide surface 8 being arranged to deflect the reflected inputlight through the first guide surface as output light, and the waveguidebeing arranged to image the light sources so that the output light fromthe light sources is directed into respective optical windows in outputdirections that are distributed laterally in dependence on the inputpositions of the light sources, wherein the waveguide 1 furthercomprises at least one surface relief feature formed either on at leastone of the first and second guide surfaces in a location adjacent theinput surface and intermediate the light emitting regions of the lightsources, and/or on the input surface intermediate the light emittingregions of the light sources.

Thus feature 510 may be a pin extending in the z-direction with width514 and height 516. Advantageously such a feature 510 may be formedduring moulding of waveguide 1. Alternatively the feature 510 could beseparately attached to waveguide 1.

The surface relief feature 510 may further be arranged to remove fromthe waveguide 1 at least some of the reflected light that is incidentthereon after reflection by the reflective end 4. The feature 510 mayhave an absorbing coating, or an absorbing layer may be arranged withthe feature 510 to reduce stray light.

Advantageously light rays 509 from the input surface, reflected from thereflective end 4 and guided by feature 10 on side 8 and side 6 may beincident on the feature 510 and extracted before reaching the inputsurface 2. Such light thus is not reflected into the waveguide from side1, so that the intensity of streaks 767 such as shown in FIG. 18 isreduced. Thus advantageously the Privacy performance may be achieved incooperation with the mechanical arrangement of the optical stack.

FIG. 23A illustrates that the feature 510 may be arranged with a width514 that is smaller or similar to height 516, to minimize guiding withinthe feature 510, and thus optimize the reduction of reflected light fromfeature 510. Advantageously the performance of the privacy mode may beincreased.

FIG. 23B illustrates that the feature 510 may be a hole rather than theprotrusion shown in FIGS. 22 and 23A. Such a hole may be black coated toreduce stray light reflections. Further the sides of the hole may beprovided with a microstructure to minimize the directionality of thestreaks 367 in privacy mode of operation.

FIGS. 24-25B are schematic diagrams illustrating front views of theinput of light into a directional waveguide and bounding regions forsurface relief features. Thus at least one surface relief feature 510 isformed on at least one of the first and second guide surfaces 6,8 of thewaveguide 1 in a location adjacent the input surface 2 and in regions500 intermediate the light emitting regions 515, 517 of the lightsources 15 a-n. 2. The location of the surface relief feature 510 may bewithin a region 500 bounded by: a portion of the input surface 2intermediate the light emitting regions of a pair of adjacent lightsources 15 a, 15 b, and a pair of intersecting notional lines 504, 506that extend from the respective edges of the light emitting regions 517,519 of the pair of light sources 15 a, 15 b that are adjacent theportion of the input surface 2, to the respective sides of thereflective end 4 that extend between the first and second guide surfaces6, 8.

Thus light sources 15 a-n may have a package material 519 and lightemitting region 515. Region 500 is formed by light rays 504, 506 fromthe edge of the region 515 to the edges of the reflective end 4, andinput surface 2.

Advantageously the features 510 do not degrade the uniformity of theoutput. By way of comparison, features outside region 500 may createlight loss such that some regions of the reflective end 4 areilluminated at lower intensity in comparison to other regions which havelight rays that do not pass through features 510.

FIGS. 25A-25B illustrate different arrangements of input surfacesarranged to optimize area for features 510 without degrading efficiencywhile advantageously achieving high uniformity of illumination.

It may be desirable to provide a directional display with uniformappearance for off-axis viewing positions in a wide angle of viewing.

FIG. 26A is a schematic diagram illustrating a perspective front view ofoff-axis ray propagation in a directional backlight for a wide viewingmode of operation and FIG. 26B is a schematic diagram illustrating aperspective front view of off-axis ray propagation in a directionalbacklight for a privacy viewing mode of operation.

In wide mode of operation groups of light sources 801, 803, 805 of array15 are all operated and an image can be seen for an off-axis viewingposition as shown in FIG. 26A. In Privacy mode of operation only group801 are operated so an image can be seen for a central viewing positionbut off-axis observers cannot see the image as shown in FIG. 26B.

It may be desirable to minimize the luminance of the off-axis image inthe Privacy mode of operation.

FIG. 26C is a schematic diagram illustrating a top view of light inputand light reflection from the input side of the directional waveguide 1in the region of groups 803, 805. Light sources of array 15 may bearranged with microstructures on the input side 2 such as described inU.S. patent application Ser. No. 15/290,543 filed Oct. 11, 2016(Attorney Ref. No. 394001) and incorporated by reference herein in itsentirety.

In a wide mode of operation the light sources in groups 803, 805 arearranged to provide input rays 802 that provide uniform illumination fora wide range of viewing positions.

However, in Privacy mode light rays 811 that reflect from the lightsource package or rays 812 that reflect from the input microstructure atthe input end create an effective illumination source. Such rayspropagate back in the waveguide 1 and provide undesirable off-axisillumination thus reducing the effectiveness of the Privacy mode ofoperation.

It may be desirable to reduce the reflectivity of features at the inputend 2 of the waveguide 2.

Modifications to the input end 2 are described in U.S. Pat. No.9,350,980 (Attorney Ref. No 317001), which is incorporated by referenceherein in its entirety. The thickness of the input end may be 0.5 mm orless. Applying light absorbing layers to such a low thickness is complexand increases cost. It may be desirable to provide a reduced costarrangement to reduce reflections.

It would be further desirable to provide stable alignment of lightsource array 15 to the input end to increase coupling efficiency and toreduce hot spot visibility.

FIGS. 26D-E are schematic diagrams illustrating side and top viewsrespectively of a strip arranged to provide reduced reflection of lightrays from an input end 2.

A directional backlight may comprise a waveguide 1 comprising first andsecond, opposed guide surfaces 6,8 for guiding light along the waveguide1 and an input surface 2 extending between the first and second guidesurfaces 6,8; an array 15 of light sources 15 a-n arranged at differentinput positions along the input surface 2 of the waveguide 1 andarranged to input input light into the waveguide 1, the light sources 15a-n having light emitting regions that are spaced apart, the waveguide 1further comprising a reflective end 4 for reflecting input light fromthe light sources 15 a-n back along the waveguide 1, the second guidesurface 8 being arranged to deflect the reflected input light throughthe first guide surface 6 as output light, and the waveguide 1 beingarranged to image the light sources 15 a-n so that the output light fromthe light sources is directed into respective optical windows 26 a-n inoutput directions that are distributed laterally in dependence on theinput positions of the light sources 15 a-n.

The directional waveguide 1 may further comprise a support 816 whichsupports the array 15 of light sources and has a portion 813 extendingpast the input surface 2 of the waveguide 1 across the first or secondguide surface of the waveguide 1.

At least one strip 815 is adhered to at least one of the first guidesurface 6 and the second guide surface 8 of the waveguide 1 andextending therealong adjacent to the input surface 2, the strip 815being arranged to reduce reflection of light rays 806 incident thereonfrom inside the waveguide 1.

In operation light rays 806 that have been reflected at the reflectiveend 4 may be incident on the strip 815. Light may propagate into thestrip 815 and the reflected ray intensity may be reduced by absorptionor scattering as will be described herein. The light ray 806 that hasbeen reflected by the input side may be incident on the strip 815 asecond time and further absorption may take place. For illustrativepurposes, the ray intensity on reflection at the input side is shownwith the same line width to illustrate intensity. However, some lightwill be transmitted and absorbed at the input end 2, with some lightreflected back into the waveguide towards the reflective end 4.

Advantageously, the rays 806 undergo losses at the strip 815, input end2 and a second time at the strip 815. Light ray 806 may beadvantageously reduced and privacy performance increased.

Further light rays directly from the light source of the array 15 may beincident on the strip 815 for a single pass. Thus the loss of inputlight rays 808 is advantageously lower than the loss of reflected lightrays 806.

The support 816 is illustrated as being on the second guide surface 8,however the strip may be on the first guide surface 6. To furtherincrease the reduction of reflected light a further strip 815 b may bearranged on the first guide surface 6. The amount of reflectionreduction may be further controlled by adjusting the widths 819, 819 bof the strips 815, 815 b. Advantageously the trade-off between inputlight loss and reflection reduction may be controlled to minimize widemode power consumption and reduce privacy image luminance.

As illustrated in FIG. 26E the light sources may have light emittingregions 515 that are spaced apart, and the strip extends along thesecond guide surface waveguide across both locations 820 adjacent to thelight emitting regions of the light sources and locations 821intermediate the light emitting regions of the light sources.

Advantageously light reflections from light sources of the array 15 andreflections from the input end may be reduced.

It may be desirable to increase the efficiency of input light whileminimizing the visibility of reflections from the input side in theregions intermediate the light sources.

FIG. 26F is a schematic diagram illustrating a top view of a strip 815and support 816 arranged on the first guiding surface 6. The strip 815may extend along only a part of the second guide surface 8 and may befor example triangular in shape for example. Advantageously reducedreflection of light rays 806 from an input end 2 may be provided whileachieving increased efficiency for input light rays 808 in comparison tothe arrangement of FIG. 26E.

FIG. 26G is a schematic diagram illustrating a top view of thearrangement of strips 815 on a waveguide 1. Said part of the first orsecond guide surface 6, 8 along which the strip 815 extends may beoffset from the center (that may be the optical axis 199) of the inputsurface 2. Advantageously the efficiency of light input at the input end2 for central light sources of the array 15 in regions 823 near theoptical axis 199 is not reduced by the strip 815. The strips 815 may bearranged in the outer regions 827 which contribute to the luminance ofthe privacy mode in off axis viewing positions. In intermediate regions825 strips 815 such as those shown in FIG. 26F may be arranged to reducereflections from the input side 2 while minimizing input light loss fromthe LEDs, thus increasing wide angle mode efficiency but reducingluminance for intermediate positions in privacy mode of operation.

The strip 815 may be an adhesive tape or may be an adhesive material.For example the strip 815 may comprise a pressure sensitive adhesive(PSA), an optically clear adhesive (OCA) tape such as 3M™ OpticallyClear Double-Sided Acrylic Adhesive Tape. Alternatively the strip 815may comprise a cured liquid crystal material such as but not limited toUV or thermally cured resin materials. The optical properties of suchmaterials will be described further herein.

It would further be desirable to achieve accurate and stable mechanicalalignment of the light sources to the input side.

The strip 815 will now be described further with reference to FIG. 26D.A directional backlight may further comprise a support 816 whichsupports the array of light sources and has a portion 813 extending pastthe input surface 2 of the waveguide 1 across the first or second guidesurface 6,8 of the waveguide 1, and wherein the at least one strip 815comprises at least one strip adhered to the support 816 and to one ofthe first guide surface 6 and the second guide surface 8 of thewaveguide 1 for holding the waveguide in position relative to the lightsources of the array 15 supported on the support 816.

Advantageously privacy mode performance may be improved, and lightsources aligned to the input of the waveguide 1 using the same strip815, further achieving reduced cost and complexity.

The strip 815 may be adhered to the support 816 and to the first orsecond guide surface 6, 8 of the waveguide 1.

A directional backlight may further comprise at least one further strip815 b provided on the other of the first guide surface 6 and the secondguide surface 8 of the waveguide 1 and extending therealong adjacent theinput surface 2, the further strip 815 b also being arranged to absorblight rays 806 incident thereon from inside the waveguide 1.

The at least one strip may comprise at least one strip 815 b adhered tothe first guide surface 6 and at least one strip 815 adhered to thesecond guide surface 8 of the waveguide 1.

The absorption of light rays 806 will now be described.

The strip 815 may be absorptive of light, whereby the strip reducesreflection of light incident thereon from inside the waveguide 1 byabsorbing that light. For example, the strip 815 b may compriseabsorptive particles in its bulk. The strip may be absorptive of lightthroughout the wavelength range of the light from the array of lightsources. For example a black pigment or dye may be incorporated in thestrip 815 b.

As illustrated by strip 815 c the strip may be transmissive of light,whereby the strip reduces reflection of light incident thereon frominside the waveguide 1 by coupling that light out of the waveguide 1.

It may be desirable to provide further control of the light extractionfrom the strip 815.

In operation, light rays that are output in a normal direction maypropagate at angle 831 within the waveguide with respect to the x axisat angles in the cone +/−20 degrees for example. Such rays may bereflected by the input to output in unwanted privacy directions. Itwould thus be desirable to reduce rays that are close to the x-axis.

The strip may have a refractive index that differs from the refractiveindex of the waveguide by no more than 0.02.

In an illustrative example, the waveguide may have a refractive index of1.50 and the strip may have a refractive index of 1.48. The criticalangle at the interface of the strip 815 and the waveguide 1 may be 80degrees, so that only +/−10 degree light cone will be maintained withinthe waveguide by the interface. Rays from the waveguide 1 that have asmall deflection in the vertical direction and output to off axisdirections may be attenuated by the strip 815 and thus the privacy modeluminance may be reduced.

It would be advantageous to minimize the complexity of the support 816.The support may be a flexible printed circuit (FPC). The FPC maycomprise the electrical connections to the array of light sources.Advantageously the cost and complexity of the support 816 may bereduced.

The support 816 may comprise an absorptive material. For example thesupport 816 may be an FPC that comprises a black solder mask layer.Light that is transmitted by strip 815 is incident on the black materialon the support and absorbed.

It may be desirable to provide heat flow path and rigidity to theassembly of light source array 15, FPC 816 and waveguide 1.

FIG. 26H is a schematic diagram illustrating in side view a directionalbacklight that further comprises a rigid holder portion 833 to which thesupport 816 is attached. The holder portion 833 may comprise a framesuch as an aluminum or steel frame material. The holder may be anodizedor black coated to reduce light scatter.

FIG. 26I is a schematic diagram illustrating in side view a directionalbacklight that illustrates that the support 816 may be a rigid holderportion 833.

The directional backlight may further comprise a resilient member 920provided behind the light sources and resiliently biasing the lightsources towards the waveguide 1 as will be described in further detailherein.

It may be desirable to provide preferential extraction of reflectedlight without the complexity of strip 815 and that can be providedduring moulding of the waveguide 1.

FIGS. 26J-K are schematic diagrams illustrating in side and top views adirectional backlight comprising diffusive light extraction regions 835,835 b that may be on the first guiding surface 6 and and/or secondguiding surface 8. Light rays 839 may be scattered onto absorber 837,837 b. Diffusing regions may for example comprise microstructuredelements that may be provided by diamond tooling or laser engraving of amoulding tool.

Privacy performance may be improved while maintaining input lightefficiency. Further complexity of assembly may be reduced. Absorbers 837may for example comprise a black solder mask on an FPC or blackenedsurface of a substrate such as a rigid holder 833.

FIGS. 26L-N are schematic diagrams illustrating in top view furtherarrangements of strips 815 and diffusing regions 835. FIG. 26Lillustrates an arrangement to reduce reflections in region 821 whileproviding some improvement from the strip 815 in region 820.

FIG. 26M illustrates that the trade-off between Privacy and inputefficiency can be adjusted by varying the width 819 of the strips 815 inthe lateral direction. Strips 815 and regions 835 may be arranged on thesame or different guiding surfaces 6, 8 of the waveguide 1.

FIG. 26N illustrates that triangular strips 815 may be provided between“Mayan” LEDs to optimize output of rays 799 while minimizing reflectionof rays 797. Mayan LEDs are typically arranged in outer regions 827 ofwaveguide 1 as illustrated in FIG. 26G and are advantageous foroptimizing wide angle uniformity. However such structures have anincreased reflectivity due to total internal reflection from theinclined surfaces and are positioned at locations to provide off axisillumination of the display. Thus the present embodiment advantageouslyimproves privacy performance while allowing efficient illumination inwide angle mode.

It may be desirable to provide the light control advantages describedabove in arrangements wherein the width of the printed circuit has asmall width.

FIG. 26P is a schematic diagram illustrating in side view a directionalbacklight that further comprises a rigid holder portion to which thewaveguide 1 is attached. Printed circuit 877 may be provided with a lowwidth that is not large enough to support sufficient width of strips815. Strips 815 c, 815 b may be arranged in contact with holder 879.Advantageously smaller width FPC may be used.

FIG. 27 is a schematic diagram illustrating in side view a directionalbacklight comprising further absorptive region 834 on the input side inthe region 821 intermediate the light sources of the array. Absorptiveregion may be provided by a tape or paint for example. Advantageouslyreflection from the input side can further be reduced and privacy modeperformance increased.

The appearance and formation of input light streaks and hotspots willnow be described.

FIG. 28A is a schematic diagram illustrating a perspective front view oflight streaking in a directional waveguide. Viewed from an off-axisviewing positions, particularly in Privacy mode of operation, streaks706 may be visibility to an observer. Such streaks are associated witheach light source of the array 15 of light sources and are inclined witha tilt that is directed from the center to a location to the near sideof the reflective end, indicating that the origin of the light streak isfor light that is propagating from the input end 2 to the reflective end4.

One origin of light streaks 706 will now be described.

FIGS. 28B is a schematic diagram illustrating in side view thepropagation of light waves in a planar waveguide. Light rays 700 may berepresented by wave fronts 708 propagating with the waveguide 1. Whenthe surfaces 6, 8 are smooth then light does not escape the waveguide 1and light streaks 706 are not created.

FIG. 28C is a schematic diagram illustrating in side view thepropagation of light waves in a waveguide comprising an extractionfeature 12. The phase step at the extraction feature creates adiffraction effect such that wavefronts 708, 709 are created that mayescape the waveguide, with an angular profile 712 that varies withdirection of the ray 712. Thus diffraction at the feature 12 may createa light loss, the light may create streak 706.

FIG. 28D is a schematic diagram illustrating in side view thepropagation of light rays in a waveguide comprising an extractionfeature. Thus the diffracted waves 708, 709 may be represented by rays714, 715.

FIG. 29 is a schematic diagram illustrating a graph of simulatedextracted luminance 716 at an extraction feature 12 against angle θ, 718of light rays within the waveguide 1. Thus at angles that are close todirect through the waveguide then very little light is diffracted by theextraction features 12. However, as the angle increases then theintensity of light extracted increases. In particular at angles close tothe critical angle at the surfaces 6, 8 make the greatest contributionto diffracted light.

Another origin of light streaks 706 will now be described.

FIG. 30A is a schematic diagram illustrating in side view the mouldingof an extraction feature. Tool 720 that may have a sharply definedextraction feature surface 722 may be used to provide an injectionmoulded waveguide 1. During the cure of the injected transparent polymermaterial, differential shrinkage 724 may occur at the sharp features ofthe tool, such that rounded cusp 724 and facet dip 726 may be formed.

FIG. 30B is a schematic diagram illustrating in side view lightextraction from a light extraction feature comprising a facet dip 726.In operation high angle light rays 728 that are incident on the facetdip may have an angular deflection that creates extraction from thewaveguide at high angle, that may create input light streaks 706.

It may be desirable to reduce the appearance of light streaks bypreferential removal of high angle light rays within the waveguide.

FIG. 31A is a schematic diagram illustrating in side view preferentialextraction of high angle light rays in a waveguide. Strip 815 may beprovided with a refractive index that differs from the refractive indexof the waveguide 1 by no less than 0.08. To continue the previousillustrative embodiment, the refractive index of the strip may be 1.42,providing a critical angle of 71° and a propagating cone angle of +/−19°for rays 730 in the waveguide 1. Higher angle rays such as shown by ray732 are extracted and may be absorbed at the support 816 for example asdescribed above.

More preferably the refractive index difference may be 0.14 or greaterand most preferably 0.19 or greater, so that the cone angle of the lightthat remains trapped in the waveguide is increased, particularly forlight that escapes in the normal direction. Such refractive index stepcan be provided for example by UV curable materials with refractiveindices in the range 1.30 to 1.35.

FIG. 31B is a schematic diagram illustrating a graph of extractedluminance at an extraction feature against angle of light rays withinthe waveguide of FIG. 31A. Thus profile 736 for light rays may besubstantially reduced, minimizing streak 706 visibility. Further lightrays 728 that are extracted by facet dip are stripped from the waveguidenear the light sources and so do not contribute to high angle lightstreaks 706.

Advantageously light streak appearance can be minimized.

It may be desirable to provide a uniform strip width 819.

FIGS. 31C-D are schematic diagrams illustrating in side view provisionof a uniform width strip. In a first step support 816 may be providedwith dips 736 and a curable low index material 733 may be provided onthe support 816 between the dips. In a second step waveguide 1 may bealigned to the array 15 and pushed against the support 816 to provide athin layer of material 733. Excess material may be squeezed into thedips 736, and thus the width 819 of the strip 815 is determined by theseparation of the dips 736. The material 733 may be cured by UVirradiation to provide strip 815. Advantageously a uniform width strip815 may be provided.

It may be desirable to remove light streaks 706 for central lightsources in region 823 while providing low reflectivity in outer regions.827

FIG. 32 is a schematic diagram illustrating in top view different stripsaligned with a waveguide 1 input side 2. In an illustrative example,outer regions 738 may comprise a 3 mm width continuous strip ofoptically clear adhesive tape with refractive index 1.50. Inintermediate region 825, triangular portions 740 may be arranged withthe same adhesive tape arranged between light sources. In inner region823 a UV cured optically clear adhesive with refractive index 1.30 maybe arranged of 0.5 mm width. Thus said part of the first or second guidesurface along which the strip extends across the center of the inputsurface.

Advantageously privacy levels, head on luminance and wide angle uniformcan be optimized.

It would further be desirable to provide identification marking forpurposes of monitoring the process conditions for fabrication of thewaveguide 1.

FIG. 33A is a schematic diagram illustrating a side view of adirectional waveguide comprising identification markings. The surfacerelief feature may thus be an identification mark 520.

FIG. 33B is a schematic diagram illustrating a front view of the inputof light into a directional waveguide and bounding regions for surfacerelief and printed identification features.

FIG. 33C is a schematic diagram illustrating a side view of adirectional waveguide comprising identification markings and furtherprotrusions. Thus the waveguide 1 further comprises at least one surfacerelief feature 570 formed on the input surface 2 intermediate the lightemitting regions of the light sources 15.

Thus the feature 520 may further by arranged in region 500.Advantageously waveguide 1 may be moulded with identification marks 520to record its design and manufacturing conditions. Such identificationmarks may create waveguide losses that improve privacy performance butdo not degrade uniformity characteristics. Such identification marks canbe arranged further in cooperation with scattering marks in region 500to increase scatter and light losses in such regions, advantageouslyimproving privacy performance by reducing cross talk from reflectedlight from side 2.

The waveguide 1 may further comprise at least one surface relief feature570 formed on the input surface 2 and intermediate the light emittingregions of the light sources 15 a-n. The feature 570 may comprisefurther identification marks 520 that may be on the surface 6, surface8, or may have surface relief in the x direction as shown in FIG. 33C.

The protrusion may thus extend into the waveguide as defined by region500 and may extend rearwards from the surface 2. Advantageously the sizeof the region 500 may be increased and the size and strength ofprotrusion 570 can be increased. Further the area for identificationmarks 520 can be increased to improve visibility and ease of providingmarks on the tool.

It may be desirable to record the metallization and waveguide 1properties by printing onto the coated waveguide 1.

FIG. 33D is a schematic diagram illustrating a top view of a metallizedreflector of a directional waveguide comprising identification markings.Such markings may record optical design, process conditions, metalconditions, manufacturer and other information including alphanumericcharacters 422 and barcodes 520 for example. Advantageously suchmarkings can be provided on the metal of the reflective end, thus do notimpact the optical performance of the waveguide 1. By providing accessports in mechanical mountings, such markings can be visible externallyto a liquid crystal module, to achieve rapid diagnosis of failure modesof the backlight during operation.

It may be desirable to provide alignment of waveguide 1 to multipleoptical elements.

FIG. 34A is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising amale waveguide surface relief alignment feature and a rear reflector.Thus the surface relief feature 510 may be a mechanical fixing feature.

The directional backlight may further comprise a rear reflector 300comprising a linear array of reflective facets 302 arranged to reflectlight from the light sources, that is transmitted through the secondguide surface 8 of the waveguide 1, back through the waveguide 1 to exitthrough the first guide surface 6, the rear reflector being said furthercomponent to which the mechanical fixing feature is fixed. A directionaldisplay device may thus comprise a directional backlight and atransmissive spatial light modulator 48 arranged to receive the outputlight from the waveguide 1, 301 and to modulate it to display an image.

FIG. 34B is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide, a rearreflector and a lightbar comprising an array of light emitting elementsand a printed circuit. FIG. 34C is a schematic diagram illustrating atop view of a lightbar comprising alignment features. The rear reflector300 may be said further component to which the mechanical fixing featureis fixed. The surface relief feature 510 may be a protrusion. Thus themechanical fixing feature 510 may be fixed to a further component of thedirectional backlight such as PCB 530 arranged to provide mounting ofarray 15 of light sources 15 a-n. PCB may have a hole or slot feature532 arranged to align to feature 510 of the waveguide. Further adhesivemay be provided in feature 532 to affix the waveguide to the mechanicalcomponents of the backlight.

FIG. 35A is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide, a rearreflector, a lightbar comprising an array of light emitting elements anda printed circuit and a rear frame 540 with hole feature 542.Advantageously further mechanical stabilization of the mechanical stackcan be achieved.

FIG. 35B is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising afemale waveguide surface relief alignment feature and a rear reflectorwith a male surface relief feature. Thus the surface relief feature 510may be a recess.

FIG. 35C is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising afemale waveguide surface relief alignment feature and lightbar with amale surface relief feature 550. Advantageously the strength of the pinfeature can be increased in comparison to the material used to form thewaveguide 1. Such feature 550 may be further coated to reducedreflectivity in privacy mode of operation.

FIG. 36 is a schematic diagram illustrating a side view of a displayapparatus comprising alignment of a directional waveguide comprising amale waveguide surface relief alignment feature and an internal framewith a female surface relief alignment feature. FIG. 37 is a schematicdiagram illustrating a side view of a display apparatus comprisingalignment of a directional waveguide comprising a female waveguidesurface relief alignment feature and an internal frame with a malesurface relief alignment feature.

Thus the alignment feature may be provided on a side 6 of the waveguidethat is opposite the side 8 of the waveguide on which the lightextraction features are formed. Advantageously the mechanical alignmentfeature 510 does not interfere with the rear reflector 300.

FIG. 38 is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector in contact with a directionalwaveguide. It may be desirable to reduce damage to the waveguide 1 andrear reflector 300 from high pressure contact of the tips of the rearreflector microstructure 302 in region 613 for example.

FIG. 39 is a schematic diagram illustrating a side view of a displayapparatus comprising a surface relief feature arranged to provideseparation of rear reflector and waveguide. Thus feature 510 maycomprise further region 511 arranged to rest of the rear reflector 300and provide gap 570 between the waveguide 1 and rear reflector 300, thusreducing damage to the waveguide 1 and rear reflector 300 duringhandling.

It may be desirable to minimize damage caused by impact and movementbetween waveguide 1 and rear reflector 300. In construction, the rearreflector microstructure 302 may be formed from a harder cross-linkedmaterial such as an acrylate than the moulded thermoplastic materialsuch as PMMA or polycarbonate that may be used to form the waveguide 1.It may be desirable to provide similar hardness materials adjacent towaveguide 1 and rear reflector 300 to reduce damage between the twocomponents.

FIG. 40 is a schematic diagram illustrating in front view a directionalwaveguide and an array of light sources arranged to provide off-axisillumination of voids. Such an arrangement is described further in U.S.Patent Publ. No. 2016/0299281 entitled “Wide angle imaging directionalbacklights” (Attorney Ref. No. 379001), incorporated herein in itsentirety. Light from light sources 15 a-n is arranged to provide directillumination of reflective side 4. Reflected light illuminatesextraction features 12 and is extracted to viewing windows 26 (notshown) as described elsewhere herein. For off-axis viewing positionsvoids are created that provide non-uniformities if not corrected. Voidscan be filled by illumination from light source 815 a-n by means ofreflection from reflector 824. Thus primary input facets 326 and draftfacets 328 may be populated by light sources 15 a-n, 815 a-nrespectively.

During thermal cycling of the display the waveguide 1 may expand andcontract differentially with respect to the lightbar comprising thearrays of light sources 15, 815. It may be desirable to limit themisalignment between light sources 15, 815 and waveguide 1 duringthermal cycling.

FIG. 41 is a schematic diagram illustrating in front view an array oflightbars for the directional waveguide of FIG. 40. Lightbars maycomprise light source arrays 15, 815 and PCB 530. The PCBs may becontinuous or may be divided into separate elements 530 a-n. Each PCB530 a-n may comprise alignment features 532 such as holes that alignwith features 510 in regions 500 of the waveguide 1. In operationrelative movement of the light sources 15 a-n, 815 a-n with respect tothe waveguide is reduced while maintaining alignment of the waveguide 1to the respective light sources. Advantageously the variation of inputcoupling efficiency and uniformity is minimized during thermal cycling.

It may be desirable to provide adhesive between the waveguide 1 andmechanical structure of the backlight.

FIG. 42 is a schematic diagram illustrating a front view of the input oflight into a directional waveguide and bounding regions for adhesionfeatures. FIG. 43 is a schematic diagram illustrating a side view of adisplay apparatus comprising attachment of a directional waveguide to amechanical arrangement comprising adhesion features.

Thus the waveguide 1 further comprises adhesive 580 provided on at leastone of the first and second guide surfaces 6,8 in a location within aregion 500 bounded by a portion of the input surface 2 intermediate thelight emitting regions of a pair of adjacent light sources 15, and apair of intersecting notional lines 504, 506 that extend from therespective edges of the light emitting regions of the pair of lightsources 15 that are adjacent the portion of the input surface 2, to therespective sides of the reflective end 4 that extend between the firstand second guide surfaces 6,8.

Thus adhesive 580 may be provided in regions 500 described elsewhereherein. Adhesive arranged outside region 500 may absorb light and createnon-uniformities and streaks in optical output. Adhesive 580 may bepressure sensitive adhesive or other types of adhesive material.Advantageously uniformity is maintained while adhesive materials areused, reducing the cost of mechanical alignment.

The moulding of a directional waveguide will now be described.

FIGS. 44-46 are schematic diagrams illustrating side views of a mouldingmethod for a directional waveguide. In a first step a mould is providedcomprising core tool 740 (comprising features for the second guidingsurface), cavity 742 740 (comprising features for the first guidingsurface), input tool 748 and Fresnel mirror tool 744. Further supporttools 750 may be provided.

After alignment of the respective tools in a first step, in a secondstep material is injected to form the waveguide 1 and cured for exampleby cooling, as illustrated in FIG. 45. In a third step the waveguide 1is extracted from the tool assembly as shown in FIG. 46 and in a fourthstep features 752 such as strips 815, identification marks and otherlayers may be provided.

It may be desirable to incorporate features within the tool to reducecost and complexity of assembly.

FIG. 47 is a schematic diagram illustrating a side view of a directionalwaveguide comprising printed identification features 754. FIGS. 48-50are schematic diagrams illustrating side views of a moulding method fora directional waveguide comprising mould inserts 756. FIG. 51 is aschematic diagram illustrating a side view of a directional waveguidecomprising moulded features from the mould inserts of FIGS. 48-50.

Advantageously features such as identification and absorptive featuresmay be incorporated in the moulding process, reducing cost andcomplexity.

Damage to the surfaces of the waveguide 1 may result in light leakagefor light that has not been reflected from the reflective end 4,reflected or refracted by the extraction features 12 and passes throughthe first guiding surface 6. Such damage may appear as white spots forexample in Privacy mode of operation for off axis viewing positions.Damage to the peaks of the rear reflector may appear as dark or whitespots.

FIG. 52A is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector 302 for a directional waveguide 1with damage artifacts. Rear reflector 302 may comprise a metallizedsurface relief structure that may have further hard protection coatingreplied. The surface relief structure may comprise a cured acrylatepolymer material for example. It would be desirable to maximize theefficiency of the rear reflector 302 by reducing the size of peaks 609.Smaller peaks 609 however provides increased pressure between thecontact points of the rear reflector 302 and waveguide 1.

Waveguide 1 that may be a relatively soft material such as PMMA orpolycarbonate for example. Pencil hardness of rear reflector 302 surfacemay be greater than 2H for example 4H, while pencil hardness ofwaveguide 1 may be less than 2H for example HB.

Force 613 may be applied to the substrate 300 of the rear reflector,providing direct contact between the two surfaces. Sharp peaks 609 incontact with waveguide 1 may form digs 601 in the material of thewaveguide 1 and may form debris 603. Further peaks 609 may be damaged toform damage regions 605 and debris 607. Debris 603 and 607 may bedeposited on surfaces of waveguide 1 and rear reflector 302.

FIG. 52B is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguideillustrating light propagation due to damage artifacts and FIG. 52C is aschematic diagram illustrating a perspective look down view of a displayapparatus comprising a rear reflector for a directional waveguideillustrating light propagation due to damage artifacts.

In operation light rays 611 may be provided for dig 601. Thus for lightpropagating from the input end 2 towards the reflective end 4 light maybe undesirably extracted, forming a bright spot 601 in the output imageas illustrated in FIG. 52C. Further, damage regions 605 on the rearreflector 302 may provide rays 617, when rays 619 may be expected, sothat black spot is provided in the output image. Debris 607 mayintroduce rays 617 providing white spots in some directions andpreventing reflected rays 621 or reflections from the rear reflector,providing dark spots.

It would be desirable to reduce the number and size of damage featuresdue to contact between the rear reflector 302 surface and the waveguide1.

It would further be desirable to reduce Moire artifacts between therepetitive extraction features 12 of the waveguide 1 and the facets ofthe rear reflector 302.

FIGS. 53A-C are schematic diagrams illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguide,further comprising an intermediate layer 600 arranged between thewaveguide 1 and rear reflector 302.

A directional backlight may comprise a waveguide 1 comprising first andsecond, opposed guide surfaces 6,8 for guiding light along the waveguide1, an input surface 2 extending between the first and second guidesurfaces 6,8 for receiving input light, and a reflective end 4 forreflecting input light from the light sources back along the waveguide1. An array 15 of light sources is arranged at different input positionsalong the input surface of the waveguide 1 and arranged to input theinput light into the waveguide 1. The first guide surface 6 is arrangedto guide light by total internal reflection and the second guide surface8 has a stepped shape comprising a plurality of extraction facets 12oriented to reflect input light from the light sources 15, afterreflection from the reflective end 4, through the first guide surface 6as output light, and intermediate regions 10 between the facets 12 thatare arranged to guide light along the waveguide 1, the waveguide 1 beingarranged to image the light sources so that the output light is directedinto respective optical windows 26 (not shown) in output directions thatare distributed laterally in dependence on the input positions of thelight sources. Further a rear reflector 302 comprising a linear array ofreflective facets arranged to reflect light from the light sources, thatis transmitted through the plurality of facets 12 of the waveguide 1,back through the waveguide 1 to exit through the first guide surface 6;and a transmissive sheet 600 arranged between the rear reflector and thesecond guide surface of the waveguide 1.

Transmissive sheet 600 may for example comprise a single planar layer602 with for example similar hardness to rear reflector 302. Sheet 600may be a glass sheet for example, or may be a polymer layer. Sheet 600may be provided with an internal diffuser effect, for example byscattering particles.

Advantageously the pressure from peaks 609 may be reduced, thusproviding reduced increase resistance to damage of the waveguide 1 andrear reflector 302. Further the separation of extraction features 12from peaks 609 of the rear reflector 302 may be increased, reducingMoire beating between the two structures.

FIG. 53B illustrates an additional layer 604 may be arranged on theupper surface of the layer 602. Thus the transmissive sheet 600comprises plural layers 602, 603, 604. The additional layer 604 mayachieve reduced damage to the waveguide 1 and/or may comprise a diffuserfunction for example that may reduce the visibility of Moire. The plurallayers of the transmissive sheet 600 include a front protective layer604 adjacent the waveguide 1, the front protective layer 604 being madeof a material that provides less damage to the waveguide 1 than thematerial of any other layer of the plural layers.

Advantageously damage to the waveguide 1 may be reduced.

As shown in FIG. 53C an additional layer 604 may be arranged on thelower surface of the layer 602. The plural layers of the transmissivesheet 600 may include a rear protective layer 602 adjacent the rearreflector 302, the rear protective layer 602 being made of a materialthat provides less damage to the rear reflector than the material of anyother layer of the plural layers.

The plural layers of the transmissive sheet 600 may include areinforcing layer 603 made of a material having a higher stiffness thanthe material than any other layer of the plural layers. Advantageouslyoptical aberrations due to distortions of the transmissive sheet 600 maybe reduced.

The reinforcing layer may have a thickness arranged to provide reducedMoire beating between the waveguide 1 and rear reflector 302.

It may be desirable to reduce the damage from the peaks of a rearreflector 302 to the surface of waveguide 1 during relative movement ofthe two elements.

FIG. 54 is a schematic diagram illustrating a side view of a displayapparatus comprising a rear reflector for a directional waveguide,wherein the rear reflector further comprises substantially coplanar flatregions 610. Such regions 610, 612 may be arranged between reflectivefacets 604, 606 in order to reduce the pressure of peaks of the rearreflector 300 onto the waveguide 1.

Advantageously robustness of the assembled device can be improved.

It may be desirable to provide efficient coupling of light from thearray 15 of light sources into the waveguide 1. In conventionalnon-directional waveguides a scattering adhesive element may be used toattach the waveguide to a substrate provided with an array of LEDsacross the whole width of the light source array.

By way of comparison in directional waveguides such adhesive mayincrease hotspot visibility and may result in increased cross talk forsome of the light sources in the array of light source. Thus althoughsome light sources may be provided with adhesive strips as describedelsewhere herein, other light sources in the array may desirably beprovided with no adhesive strip between light source and array. It isdesirable to provide high coupling efficiency between light sources andwaveguides without the use of adhesives.

It would be further desirable to minimize the visibility of hotspots dueto misalignment of the array 15 light sources and input end 2 of thewaveguide 1. It would be further desirable to provide a thermal pathfrom an array 15 of light sources to a frame to provide reduced junctiontemperature during operation.

FIG. 55 is a schematic diagram illustrating a side view of misalignmentof a light source 15 a comprising an LED package 861 with light emittingregion 515 with a waveguide 1 input side 2. Longitudinal misalignment860 and lateral misalignment 862 may provide efficiency loss asillustrated in FIG. 56 which is a schematic graph illustrating relativecoupling efficiency 864 of light into a waveguide 1 correspondinglongitudinal misalignment 860.

In the illustrative embodiment of FIG. 56 a waveguide 1 with height 0.55mm is aligned to an LED with height of the emitting aperture 515 of 0.45mm and Lambertian emission profile. Light that is not incident on theinput side 2 is assumed to be lost. A zero misalignment 862 is assumed(i.e. the LED is aligned in the z direction) and the misalignment 860 isadjusted. Desirably a coupling efficiency of greater than 95% isdesirable providing a maximum desirable longitudinal displacement ofapproximately 40 microns. To achieve such alignment over the entirewidth of the array 15 may increase cost and complexity of the lightbaralignment system and impact waveguide yield, increasing cost.

It may be desirable to increase coupling efficiency at reduced cost.

FIG. 57 is a schematic diagram illustrating a side view of misalignmentof an LED with a valve input side comprising additional reflectiveelements 866, 870. Reflective elements may be provided by reflectivefilms, or may be coatings on support means. FIG. 58 is a schematic graphillustrating relative coupling efficiency 864 of light into a waveguide1 further comprising additional reflective elements 866, 870 withreflectivity 872 and longitudinal misalignment 860. In comparison toFIG. 56, coupling efficiency may be improved. However, insertion ofreflective elements 866, 870 may be complex and costly.

It may be desirable to provide increased coupling efficiency in adirectional backlight in light sources that are desirably not whollyaligned by attachment to adhesive strips or other adhesive elements.

FIG. 59A is a schematic diagram illustrating side view of alignment ofan LED array 15 with a waveguide 1 in a first step. A directionalbacklight may comprise a waveguide 1 comprising first and second,opposed guide surfaces 6, 8 for guiding light along the waveguide 1 andan input end 2 comprising an input surface extending between the firstand second guide surfaces 6,8; an array of light sources 15 a-n arrangedat different input positions along the input end 2 of the waveguide 1and arranged to input input light into the waveguide 1, the lightsources 15 a-n having light emitting regions that are spaced apart. Thewaveguide may further comprise a reflective end 4 for reflecting inputlight from the light sources back along the waveguide 1, the secondguide surface 8 being arranged to deflect the reflected input lightthrough the first guide surface 6 as output light, and the waveguide 1being arranged to image the light sources 15 a-n so that the outputlight from the light sources 15 a-n is directed into respective opticalwindows 26 in output directions that are distributed laterally independence on the input positions of the light sources.

FIG. 59B is a schematic diagram illustrating a side view of alignment ofan LED array 15 with a waveguide 1 in a second step.

A holder portion 923 may be provided extending across the light sources15 a-n and the waveguide 1, the holder portion 923 holding the lightsources 15 a-n and the waveguide 1 in position relative to each other.Further a resilient member 920 may be provided behind the light sources15 a-n and resiliently biasing the light sources 15 a-n towards theinput end 2 of the waveguide 1. The directional backlight may thusfurther comprise a stop 921 extending from the holder portion 923 behindthe resilient member 920, the resilient member 920 engaging the stop921. The stop 921 may be an integral part of the holder portion 923 asillustrated for example in FIG. 60A.

During assembly a force 925 may be applied to contact the waveguide 1 tothe light source array 15 across the lateral direction, with theresilient member providing a resistant force that may vary across thearray of light sources.

The support 816 may be a printed circuit, the printed circuit may be aflexible printed circuit. Thus the support 816 may be provided by aflexible printed circuit to which the light sources are soldered.

The resilient member 920 may for example be a sponge material that maybe attached to the support 816 and may extend beyond behind the support816.

Advantageously, each light source of the array of light sources may bealigned with respect to the input to the waveguide 1 and couplingefficiency improved. An adhesive may not be inserted between thewaveguide 1 and support 816, that may reduce light losses in comparisonto arrangements using adhesives.

FIGS. 60A-B are schematic diagrams illustrating side views of alignmentof an illumination assembly with a mechanical support further comprisingadhesive layers.

A support 923 may be provided which supports the array of light sources,the support being attached to the holder portion 530. The stop 921 maybe an integral part of the support 923 as illustrated in FIG. 60B forexample.

The support 816 may have a portion extending past the input end 2 of thewaveguide 1 across the first guide surface 6 or second guide surface 8of the waveguide 1, and the directional backlight may further compriseat least one strip 815 adhered to at least one of the first guidesurface and the second guide surface of the waveguide and extendingtherealong adjacent to the input surface 2, the strip 815 being arrangedfor holding the waveguide 1 in position relative to the light sources 15supported on the support 816, the light-absorptive adhesive strip 815extending along the second guide surface 8 of waveguide 1 adjacent tothe input end 2.

FIGS. 61A-B are schematic diagrams illustrating a side view of alignmentof an illumination assembly with a slotted mechanical support. FIG. 61Aillustrates that a holder portion 923 may be provided as a slotted frame922 and arranged to receive the light source array 15, support 816 andwaveguide 1. FIG. 61B illustrates that the rear of the slotted frame 922may act as a stop for the resilient member.

Advantageously the slotted frame 922 may conveniently provide a supportfor assembly of the directional backlight into a frame for a directionaldisplay.

It may be desirable to provide alignment of the waveguide 1 and lightsource array 15 in the normal direction (z-direction).

FIGS. 62A-B are schematic diagrams illustrating a side view of alignmentof an illumination assembly with a sprung mechanical support. Adhesivelayer 925 such as an adhesive tape may be provided to attach the support816 to a frame 924 that may be sprung. Recessed feature 926 may beprovided to apply a vertical force to the waveguide 1 against thesupport 816 such that alignment is achieved in the normal (z-axis)direction. Advantageously coupling efficiency may be increased.

It would be advantageous to reduce possible damage to the waveguide 1during assembly.

FIGS. 63A-C are schematic diagrams illustrating a side view of alignmentof an illumination assembly with a clipped mechanical support. In afirst step stop 921 of holder portion 930 may be arranged to providelongitudinal alignment of the light sources of the array 15 as describedabove. In a second step, clip 932 may be attached to provide clampingforce in the normal (z-axis) direction.

Advantageously possible damage during assembly may be reduced.

It may be advantageous to further combine the resilient member into theframe of the directional backlight.

FIGS. 64A-B are schematic diagrams illustrating top and side viewsrespectively of alignment of an LED array with a waveguide comprising adeformable mechanical support as a holder portion 923. Frame 940 may forexample be formed in a bendable metal such as aluminum. Resilient member920 may be provided by bendable lugs 941 that may be provided in theframe 940 and used to push the light sources of the array 15 against theinput side 2 of the waveguide 1.

Advantageously the resilient member is incorporated in the frame 940,reducing cost and complexity.

In some arrangements side mirrors 827 may be provided on the sides 24,26 of the waveguide 1. It may be desirable to provide mechanicalalignment of the side mirrors 827 to the sides 24, 26 of the waveguide1.

FIGS. 65A-B are schematic diagrams illustrating top and side viewsrespectively of alignment of side mirror with a waveguide comprising adeformable mechanical support. Side mirrors 827 may comprise in additionto a reflective surface a substrate 824 and support 825. Lugs 924 inframe 940 may provide resilient member 920 to apply force 944 betweenthe side mirror 827 and mirror.

Advantageously leakage of light at the side mirrors and couplingefficiency may be optimized.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

What is claimed is:
 1. A directional backlight comprising: a waveguidecomprising first and second, opposed guide surfaces for guiding lightalong the waveguide and an input surface extending between the first andsecond guide surfaces; and an array of light sources arranged atdifferent input positions along the input surface of the waveguide andarranged to input input light into the waveguide, the light sourceshaving light emitting regions that are spaced apart, the waveguidefurther comprising a reflective end for reflecting input light from thelight sources back along the waveguide, the second guide surface beingarranged to deflect the reflected input light through the first guidesurface as output light, and the waveguide being arranged to image thelight sources so that the output light from the light sources isdirected into respective optical windows in output directions that aredistributed laterally in dependence on the input positions of the lightsources, wherein the waveguide further comprises at least one surfacerelief feature formed either on at least one of the first and secondguide surfaces in a location adjacent the input surface and intermediatethe light emitting regions of the light sources, and/or on the inputsurface intermediate the light emitting regions of the light sources. 2.A directional backlight according to claim 1, wherein said location ofthe surface relief feature is within a region bounded by: a portion ofthe input surface intermediate the light emitting regions of a pair ofadjacent light sources, and a pair of intersecting notional lines thatextend from the respective edges of the light emitting regions of thepair of light sources that are adjacent the portion of the inputsurface, to the respective sides of the reflective end that extendbetween the first and second guide surfaces.
 3. A directional backlightaccording to claim 1, wherein the surface relief feature is a mechanicalfixing feature.
 4. A directional backlight according to claim 3, whereinthe mechanical fixing feature is fixed to a further component of thedirectional backlight.
 5. A directional backlight according to claim 4,further comprising a rear reflector comprising a linear array ofreflective facets arranged to reflect light from the light sources, thatis transmitted through the plurality of facets second guide surface ofthe waveguide, back through the waveguide to exit through the firstguide surface into said optical windows, the rear reflector being saidfurther component to which the mechanical fixing feature is fixed.
 6. Adirectional backlight according to claim 1, wherein the surface relieffeature is a protrusion.
 7. A directional backlight according to claim1, wherein the surface relief feature is a recess.
 8. A directionalbacklight according to claim 1, wherein the surface relief feature isarranged to remove from the waveguide at least some of the reflectedlight that is incident thereon after reflection by the reflective end.9. A directional backlight according to claim 1, wherein the surfacerelief feature is an identification or data mark.
 10. A directionalbacklight according to claim 1, wherein the input surface is an end ofthe waveguide opposite to the reflective end.
 11. A directionalbacklight according to claim 1, wherein the input surface is a sidesurface of the waveguide extending away from the reflective end.
 12. Adirectional backlight according to claim 1, wherein the first guidesurface is arranged to guide light by total internal reflection and thesecond guide surface comprises a plurality of light extraction featuresoriented to direct light guided along the waveguide in directionsallowing exit through the first guide surface as the output light andintermediate regions between the light extraction features that arearranged to guide light along the waveguide.
 13. A directional backlightaccording to claim 12, wherein the second guide surface has a steppedshape in which said light extraction features are facets between theintermediate regions.
 14. A directional waveguide according to claim 12,wherein the light extraction features have positive optical power in adirection between sides of the waveguide that extend between the firstand second guide surfaces and between the input end and the reflectiveend.
 15. A directional waveguide according to claim 1, wherein thereflective end has positive optical power in a direction extendingbetween sides of the waveguide that extend between the first and secondguide surfaces and between the input end and the reflective end.
 16. Adirectional display device comprising: a directional backlight accordingto claim 1; and a transmissive spatial light modulator arranged toreceive the output light from the waveguide and to modulate it todisplay an image.
 17. A directional display apparatus comprising: adirectional display device according to claim 16; and a control systemarranged to control the light sources.
 18. A directional backlightcomprising: a waveguide comprising first and second, opposed guidesurfaces for guiding light along the waveguide and an input surfaceextending between the first and second guide surfaces; and an array oflight sources arranged at different input positions along the inputsurface of the waveguide and arranged to input input light into thewaveguide, the light sources having light emitting regions that arespaced apart, the waveguide further comprising a reflective end forreflecting input light from the light sources back along the waveguide,the second guide surface being arranged to deflect the reflected inputlight through the first guide surface as output light, and the waveguidebeing arranged to image the light sources so that the output light fromthe light sources is directed into respective optical windows in outputdirections that are distributed laterally in dependence on the inputpositions of the light sources, wherein the waveguide further comprisesadhesive provided on at least one of the first and second guide surfacesin a location within a region bounded by: a portion of the input surfaceintermediate the light emitting regions of a pair of adjacent lightsources, and a pair of intersecting notional lines that extend from therespective edges of the light emitting regions of the pair of lightsources that are adjacent the portion of the input surface, to therespective sides of the reflective end that extend between the first andsecond guide surfaces.
 19. A directional backlight comprising: awaveguide comprising first and second, opposed guide surfaces forguiding light along the waveguide and an input surface extending betweenthe first and second guide surfaces; an array of light sources arrangedat different input positions along the input surface of the waveguideand arranged to input input light into the waveguide, the light sourceshaving light emitting regions that are spaced apart, the waveguidefurther comprising a reflective end for reflecting input light from thelight sources back along the waveguide, the second guide surface beingarranged to deflect the reflected input light through the first guidesurface as output light, and the waveguide being arranged to image thelight sources so that the output light from the light sources isdirected into respective optical windows in output directions that aredistributed laterally in dependence on the input positions of the lightsources; and at least one strip adhered to at least one of the firstguide surface and the second guide surface of the waveguide andextending therealong adjacent to the input surface, the strip beingarranged to reduce reflection of light incident thereon from inside thewaveguide.
 20. A directional backlight according to claim 19, whereinthe light sources have light emitting regions that are spaced apart, andthe strip extends along at least one of the first guide surface and thesecond guide surface across both locations adjacent to the lightemitting regions of the light sources and locations intermediate thelight emitting regions of the light sources.
 21. A directional backlightaccording to claim 19, wherein the strip extends along only a part of atleast one of the first guide surface and the second guide surface.
 22. Adirectional backlight according to claim 21, wherein said part of atleast one of the first guide surface and the second guide surface alongwhich the strip extends is offset from the center of the input surface.23. A directional backlight according to claim 21, wherein said part ofat least one of the first guide surface and the second guide surfacealong which the strip extends is across the center of the input surface.24. A directional backlight according to claim 19, wherein the strip isan adhesive tape.
 25. A directional backlight according to claim 19,wherein the strip is adhesive material.
 26. A directional backlightaccording to claim 19, further comprising a support which supports thearray of light sources and has a portion extending past the inputsurface of the waveguide across the first guide surface or the secondguide surface of the waveguide, and wherein the at least one stripcomprises at least one strip adhered to the support and to one of thefirst guide surface and the second guide surface of the waveguide forholding the waveguide in position relative to the light sourcessupported on the support.
 27. A directional backlight according to claim19, wherein the strip is adhered to the support and to one of the firstguide surface and the second guide surface of the waveguide.
 28. Adirectional backlight according to claim 19, wherein the strip isadhered to the support and to the first or second guide surface of thewaveguide.
 29. A directional backlight according to claim 26, furthercomprising at least one further strip provided on the other of the firstguide surface and the second guide surface of the waveguide andextending therealong adjacent the input surface, the further strip alsobeing arranged to absorb light incident thereon from inside thewaveguide.
 30. A directional backlight according to claim 19, whereinthe at least one strip comprises at least one strip adhered to the firstguide surface of the waveguide and at least one strip adhered to thesecond guide surface of the waveguide.
 31. A directional backlightaccording to claim 19, wherein the strip is absorptive of light, wherebythe strip reduces reflection of light incident thereon from inside thewaveguide by absorbing that light.
 32. A directional backlight accordingto claim 31, wherein the strip is absorptive of light throughout thewavelength range of the light from the array of light sources.
 33. Adirectional backlight according to claim 19, wherein the strip istransmissive of light, whereby the strip reduces reflection of lightincident thereon from inside the waveguide by coupling that light out ofthe waveguide.
 34. A directional backlight according to claim 33,wherein the strip has a refractive index that differs from therefractive index of the waveguide by no more than 0.02.
 35. Adirectional backlight according to claim 33, wherein the strip has arefractive index that differs from the refractive index of the waveguideby no less than 0.08.
 36. A directional backlight according to claim 19,wherein the support is a flexible printed circuit.
 37. A directionalbacklight according to claim 19, further comprising a rigid holderportion to which the support is attached.
 38. A directional backlightaccording to claim 19, wherein the support is a rigid holder portion.39. A directional backlight according to claim 19, further comprising aresilient member provided behind the light sources and resilientlybiasing the light sources towards the waveguide.
 40. A directionalbacklight according to claim 19, wherein the first guide surface isarranged to guide light by total internal reflection and the secondguide surface comprises a plurality of light extraction featuresoriented to direct light guided along the waveguide in directionsallowing exit through the first guide surface as the output light andintermediate regions between the light extraction features that arearranged to guide light along the waveguide.
 41. A directional backlightaccording to claim 40, wherein the second guide surface has a steppedshape in which said light extraction features are facets between theintermediate regions.
 42. A directional waveguide according to claim 40,wherein the light extraction features have positive optical power in adirection between sides of the waveguide that extend between the firstand second guide surfaces and between the input surface and thereflective end.
 43. A directional waveguide according to claim 19,wherein the reflective end has positive optical power in a directionextending between sides of the waveguide that extend between the firstand second guide surfaces and between the input end and the reflectiveend.
 44. A directional backlight according to claim 19, wherein theinput surface is an end of the waveguide opposite to the reflective end.45. A directional display device comprising: a directional backlightaccording to claim 19; and a transmissive spatial light modulatorarranged to receive the output light from the waveguide and to modulateit to display an image.
 46. A directional display apparatus comprising:a directional display device according to claim 45; and a control systemarranged to control the light sources.
 47. A directional backlightcomprising: a waveguide comprising first and second, opposed guidesurfaces for guiding light along the waveguide and an input endcomprising an input surface extending between the first and second guidesurfaces; an array of light sources arranged at different inputpositions along the input end of the waveguide and arranged to inputinput light into the waveguide, the light sources having light emittingregions that are spaced apart, the waveguide further comprising areflective end for reflecting input light from the light sources backalong the waveguide, the second guide surface being arranged to deflectthe reflected input light through the first guide surface as outputlight, and the waveguide being arranged to image the light sources sothat the output light from the light sources is directed into respectiveoptical windows in output directions that are distributed laterally independence on the input positions of the light sources; a holder portionextending across the light sources and the waveguide, the holder portionholding the light sources and the waveguide in position relative to eachother; and a resilient member provided behind the light sources andresiliently biasing the light sources towards the input end of thewaveguide.
 48. A directional backlight according to claim 47, furthercomprising a stop extending from the holder portion behind the resilientmember, the resilient member engaging the stop.
 49. A directionalbacklight according to claim 48, wherein the stop is an integral part ofthe holder portion.
 50. A directional backlight according to claim 47,further comprising a support which supports the array of light sources,the support being attached to the holder portion.
 51. A directionalbacklight according to claim 50, wherein the support is a printedcircuit.
 52. A directional backlight according to claim 51, wherein theprinted circuit is a flexible printed circuit.
 53. A directionalbacklight according to claim 50, wherein the support has a portionextending past the input end of the waveguide across the second guidesurface of the waveguide, and the directional backlight furthercomprises a light-absorptive adhesive strip adhered to the support andto the second guide surface of the waveguide for holding the waveguidein position relative to the light sources supported on the support, thelight-absorptive adhesive strip extending along the second guide surfacewaveguide adjacent to the input end.
 54. A directional backlightaccording to claim 47, wherein the first guide surface is arranged toguide light by total internal reflection and the second guide surfacecomprises a plurality of light extraction features oriented to directlight guided along the waveguide in directions allowing exit through thefirst guide surface as the output light and intermediate regions betweenthe light extraction features that are arranged to guide light along thewaveguide.
 55. A directional backlight according to claim 54, whereinthe second guide surface has a stepped shape in which said lightextraction features are facets between the intermediate regions.
 56. Adirectional waveguide according to claim 54, wherein the lightextraction features have positive optical power in a direction betweensides of the waveguide that extend between the first and second guidesurfaces and between the input end and the reflective end.
 57. Adirectional waveguide according to claim 47, wherein the reflective endhas positive optical power in a direction extending between sides of thewaveguide that extend between the first and second guide surfaces andbetween the input end and the reflective end.
 58. A directionalbacklight according to claim 47, wherein the input surface is an end ofthe waveguide opposite to the reflective end.
 59. A directional displaydevice comprising: a directional backlight according to claim 47; and atransmissive spatial light modulator arranged to receive the outputlight from the waveguide and to modulate it to display an image.
 60. Adirectional display apparatus comprising: a directional display deviceaccording to claim 59; and a control system arranged to control thelight sources.
 61. A directional backlight comprising: a waveguidecomprising first and second, opposed guide surfaces for guiding lightalong the waveguide, an input surface extending between the first andsecond guide surfaces for receiving input light, and a reflective endfor reflecting input light from the light sources back along thewaveguide; an array of light sources arranged at different inputpositions along the input surface of the waveguide and arranged to inputthe input light into the waveguide, wherein the first guide surface isarranged to guide light by total internal reflection and the secondguide surface has a stepped shape comprising a plurality of extractionfacets oriented to reflect input light from the light sources, afterreflection from the reflective end, through the first guide surface asoutput light, and intermediate regions between the facets that arearranged to guide light along the waveguide, the waveguide beingarranged to image the light sources so that the output light is directedinto respective optical windows in output directions that aredistributed laterally in dependence on the input positions of the lightsources; a rear reflector comprising a linear array of reflective facetsarranged to reflect light from the light sources, that is transmittedthrough the plurality of facets of the waveguide, back through thewaveguide to exit through the first guide surface; and a transmissivesheet arranged between the rear reflector and the second guide surfaceof the waveguide.
 62. A directional waveguide according to claim 61,wherein the transmissive sheet comprises plural layers.
 63. Adirectional waveguide according to claim 62, wherein the plural layersinclude a rear protective layer adjacent the rear reflector, the rearprotective layer being made of a material that provides less damage tothe rear reflector than the material of any other layer of the plurallayers.
 64. A directional waveguide according to claim 62, wherein theplural layers include a front protective layer adjacent the waveguide,the front protective layer being made of a material that provides lessdamage to the waveguide than the material of any other layer of theplural layers.
 65. A directional waveguide according to claim 62,wherein the plural layers include a reinforcing layer made of a materialhaving a higher stiffness than the material than any other layer of theplural layers.
 66. A directional waveguide according to claim 61,wherein the extraction facets are laterally curved have positive opticalpower in a direction between sides of the waveguide that extend betweenthe first and second guide surfaces.
 67. A directional waveguideaccording to claim 61, wherein the reflective end has positive opticalpower in a direction extending between sides of the waveguide thatextend between the first and second guide surfaces.
 68. A directionalbacklight according to claim 61, wherein the input surface is an end ofthe waveguide opposite to the reflective end.
 69. A directionalbacklight according to claim 61, wherein the input surface is a surfaceof a side of the waveguide extending away from the reflective end.
 70. Adirectional display device comprising: a directional backlight accordingto claim 61; and a transmissive spatial light modulator arranged toreceive the output light from the waveguide and to modulate it todisplay an image.
 71. A directional display apparatus comprising: adirectional display device according to claim 70; and a control systemarranged to control the light sources.